Steve Forbes, who actually championed flat tax as a Presidential candidate, had a figure of 17% I think. That for some unknown reason seems like a good number. Everyone could live with the extra 3 percent. Call it a tip. But that's it - lock it in and come up with a consistent plan for borrowing money - financing future productivity - from "the future". You can make long range plans if you know that the tax burden is going to be what it is, regardless. So it would have to go into the Constitution.