While the current post will only deal with an overall tax rate, and a simplified one at that, it should give a good insight to how both political ideologies should be considered when creating it.
I. The Extreme Liberal Viewpoint
I don't know how many times I've heard something to the effect of "oh - we need more money, let's raise taxes!" The assumption is that people will always be willing to pay a higher marginal tax rate and thus the government will always make more money. Of course there are ramifications which are ignored, however we'll get to those in just a bit.
For the time being, let's look at the amount of money the government would theoretically take in from a single average household that just barely made the top marginal income bracket in 2009 ($372,950) as a function of possible taxation amounts from 0% - 95%:

I've also included how far the household income is above a theoretical "just-scrape-by" amount of $40,000 which will be used and described later. As you can see though - through a strictly liberal viewpoint, The amount of money the government takes in is in direct, unchanging proportion to the tax rate.
This obviously overlooks some major factors.
II. The Extreme Conservative Viewpoint
Of course there are two sides to every coin - and the conservative side taken alone is just as harmful. I'm sure if you've made it this far reading this post, you've heard the Reganomic viewpoint that high tax rates stifle innovation and lowering them will always bring additional revenue for the government. The theory is that when tax rates are lowered - more people with capital to burn (i.e. our top income bracket) will have the desire to invest in business (which increases the working capital of the business and thus the output potential - of which the profit is then taxable) or create their own business with the same effect.
While there has been no study or poll to my knowledge on the highest tax rate an average person would consider creating a business under, I think the question itself would have significant problems in practice for the following reasons in order from what I perceive to be the highest prevalence to the lowest:
- The average conservative with no intention of ever running a business would probably tend to be resistant to a much lower tax rate than what a true conservative running a business would tolerate. This would probably be the most prevalent problem as, in some cases, it could also serve as a psychological means to justify why they themselves haven't started a business.
- The average liberal with no intention of ever running a business would probably tend to agree to a much higher tax rate than what is sustainable or practical in reality. This would probably be the second-most prevalent problem in polling as it would serve to theoretically atone for significant societal earnings gaps, although I think the rate would be very close to the #1 reason.
- The average conservative running a business would probably be more resistant to changes in tax rate in theory than reality, most likely going as far as saying something to the effect of "if there's another increase, I don't think (I / my business) could handle it" when in reality they could / would be willing to handle at least slightly more significant jumps. While I rank this third in prevalence, I feel it's an order of magnitude lower in significance than the above two.
- The average liberal running a business would probably be less resistant in theory to changes in tax rate than reality claiming willingness to give more than what their business can truly handle. This is probably the least prevalent of all being another order of magnitude less than the #3 reason simply because liberals running businesses in the top tax bracket tend to be more moderate economically.

I imagine the "dropout" rate of business creation is that of a bell curve, giving the above graph of the number of overall people willing to create or invest in a business as a % of total people capable of creating or investing in a business.
As you can see, I have placed the 50/50 mark for willingness at the 45% marginal tax bracket - meaning that if you were to increase personal income on the average business owner by 15% more than current, around 1/3rd of current businesses would disappear. I've also placed the 90% tax brackets and higher at 0% desire because the amount of tax taken places the owner below our theoretical "just scrape by" line, and the 90% desire rate is maintained above 15% tax rate as income becomes more and more expendable as more is earned.
Of course it's true that the lower the tax rate, the more people want to invest, but does that always mean more money for the government?
III. Combining The Two Ideologies
So how do we combine the two ideologies? It's actually quite simple. We'll multiply the amount of money the government would earn if everyone capable was still interested in investing under any tax amount by our modified amount based on desire. If you do so, we get the following government income graph which compensates for both ideologies:

As you can see - both ideologies are right to a certain extent. If the marginal tax rate is low enough, increasing it will bring more revenue - partially affirming the extreme liberal viewpoint, but returns are diminishing and eventually negative. The same is true about the extreme conservative viewpoint. If tax rates are too high - a reduction will influence enough people to succumb to their capitalist urges to make up for tax revenue otherwise "lost," but again with diminishing returns.
IV. Is Government Revenue The Best Measure For Ideal Tax Rate
While I wouldn't say it's the absolute best measure, it is the single best tradeoff between allowing capitalistic development and growth while allowing government to help the maximum number of people. While the government does not have the best track record on efficient spending, it does provide many essential services such as defense, police, fire, hospitals, schools, post, and it has many services to keep the minimum quality of life high for citizens.
As I said above, this example is far simplified. There are factors which influence the "ideal" tax rate in both liberal and conservative directions.
a. Factors Influencing "Ideal" Tax Rate in a Liberal Direction
All of the above assumptions exclude any government help to individuals and businesses. In reality there are a number of programs which help business such as startup credits, green credits, and similar programs. The above would make some business more likely to continue at a higher tax rate.
Another factor ignored is the trickle-up economics factor. A lot of money goes towards low-income families and individuals. The government money used goes back into the economy to purchase goods and services, potentially netting more money for companies as the velocity of money and GDP is theoretically increased. If trickle up economics play a slightly bigger role than trickle down, the ideal tax rate is actually slightly above the government maximum earning potential.
Inflationary concerns are another potential problem. Tax rates could theoretically get low enough that inflation is a bigger concern. This would again be reason to keep tax rates above the government's maximum earning potential.
a. Factors Influencing "Ideal" Tax Rate in a Conservative Direction
Tax credits generally do not favor larger businesses. Usually the larger the business, the less of a chance at tax credits, potentially stifling further investment.
Trickle-Down factors would provide that the GDP be increased (again in theory) as business pay people who then purchase products in the economy. If this is the stronger of the two forces, a lower than government maximum earning tax rate should be set.
Market stagnation concerns from this side are a problem. Theoretically, tax rates could get high enough that production drops, less people buy / get paid, making people buy less, etc.
All of these are mirror but opposite ideological concerns to those on the opposite side of the spectrum.
V. Additional notes
I want to mention, for someone earning $372,950, only one dollar of what they pay is currently taxed at 35%, Because all of the tax brackets are marginal, the total tax as a percentage of income is currently only around 29%. The graph above is a simplified "overall" tax rate which would mean it's likely we are currently below the "ideal" mark as specified above.
There is also a limited body of real-world evidence to back some of these assertions up. Under President Carter, the top marginal tax bracket was 70% ($372,950 adjusted for inflation at that time had a total rate around 60%) and that was changed to a marginal tax rate of 25% under Regan ($372,950 adjusted for inflation at that time had a total rate around 20%.) You can see from the graph of our assumptions, that represents a fairly large net increase in government revenue - which did happen.
Likewise, the Clinton era top marginal rate of 39.6% ($372,950 adjusted for inflation had about a total tax rate of around 35%) adjusted to the Bush top marginal tax rate of 35% (around 29% total tax rate) represents a small decrease in government revenue which was also reflected in reality.
While there are other extenuating factors, I suspect the curve I created is at least fairly close to reality.
VI. Do You Have A Point?
Indeed I do. The above example is to show that in reality, both ideologies co-exist. In both extremes, the government goes broke and ceases to exist while the economy spins out of control. For the conservative extreme - see Mexico.
For the liberal extreme - see Russia.
I also wanted to distinctly point out that we are currently FAR from an extreme. I have heard many shouts of "socialism" from the extreme right in the current climate when, instead we are much closer to ideal capitalism by balancing quality of life for low income and earning potential for investors and high income people.
