Depends on the percent rate, of course.
The big trick here is removing the progressive tax, meaning, if you’re broke you don’t pay anything - but if you’re rich, you pay damn near everything. Those in the middle probably would not see a huge change to our wallet, but we’d see a huge change to our politics.
Progressive, “wealth redistribution”, whatever you want to call it, is not good for anyone, and is part of what led us down this road in the first place.
The flat tax is not as good as the so-called “fair tax” which is a consumption tax - it’s a percentage applied to goods purchased/sold, rather than based on income; but BOTH are “friendlier” than the current progressive tax. With a consumption tax, you determine how much tax you pay by how much crap you buy. Less crap, less taxes.
The dilemma is how to combat the growing disparity of the rich and 'the 99%'. I think most people do not want the aid of government but they do want a job to be able to afford a home and to provide for their families' health when needed and the education of their children. A system that builds more and more poor,[ who vote accordingly], is a problem in a democracy. And that is no good either.