I have two taxpayers, Roy the rich ($1000 / month) and Peter the poor ($100 / month). Today we will have a 10% flat tax. Peter pays $10. Roy pays $100. The total tax take is $110 and Roy is paying 91% of it.
If we imagine a 'comfortable' lifestyle costs $200/month then there is an argument against using this flat tax system, but it is difficult to escape from the fact that Roy is doing almost all the heavy lifting from the who-is-funding-society perspective. Even if Roy has a very good team of accountants and is managing to only pay 5% of his income in tax (grossly unfair!) he would still be paying 83% of the total tax take (50 / 60). If we use percentages, the rich are going to be paying for most of what the government does, whether they pay personally or through corporate tax.
Most people overestimate how profitable companies are, the economics of business tend to squeeze margins. I've seen a lot of arguments in Australia whinging that not enough taxes are paid by companies like Quantas because they don't understand concepts like depreciation and carrying losses forward. Those concepts are quite fair once it is accepted that maybe assets take a few years life in them before they need to be replaced.
EDIT Bit slow this morning, I figured out what you meant by proportional basis.