Yes. They would add up all they made plus subtract all they lost and pay on the remainder. Now there's going to be stocks, bonds , interest, as well as many other things which will have to be considered "income'.
The flat tax proposals I have seen eliminate the corporate income tax. The theory is that it is a double tax, since corporations distribute their profits (ultimately) to owners after those earnings are taxed, and then the owners have to pay tax on the dividends.
On a different point the economic argument for a graduated marginal tax rate system is not that rick people "owe society" more than poor ones, it's that rich people value money less than poor ones. Let's say you earn $20 million and the government takes half. You still earn $10 million. The difference in your lifestyle and overall happiness is probably not that great. You are a multi-millionaire either way. If you each $200,000 a year and the government takes half, that is a bigger hit. At $100K per year, you are still doing pretty well, but major medical expenses, college tuitions, buying a new car, these are things you might require debt or savings to be able to afford, when on $200K you'd have had more of a cushion If you only earn $40,000, though and the government takes half, you only earn $20,000 that year. The difference in your lifestyle and general level of satisfaction is likely to be far more noticeable. Yet both paid 50%.
Even the flat tax people implicitly acknowledge this economic concept--the "diminishing marginal utility of money"--which is why no one ever proposes a true flat tax...just a *less* graduated one. When you let people earning less than $30,000 pay $0, that is a 0% tax rate, so there are two, rather than 5-6, tax rates in the "flat tax" proposals.
But is two rates are good, why not three to let the $75K-$300K income bracket off the hook mentioned above?
There is some notion that a flat tax is "simpler" but the rates in question affect only Section 1 of the tax code. The other 500,000 pages would remain unchanged. That's okay, because the flat tax people want to chuck those. But why not keep graduated rates and chuck all the other pages then? In short, there is nothing inherently much simpler about a flat tax.
Also, who here is ready to give up their child tax credit, the deduction for local income taxes paid, the mortgage interest deduction, education costs deductions...flat taxers want to eliminate all of those. Every deduction has a constituency that would fight to keep it because (they feel) "but that is we NEED."
Meanwhile, it has to be admitted that the constituency most likely to save its deductions and loopholes are the wealthy, because they just have more pull in the political realm.