A true flat tax is just that, deductions allow you to game the system. Many states have a gross receipts tax, including my state of Washington (and no income tax on individuals). As a business owner I pay 1.5% of my gross income (service industry). The tax is different for different industries. It ensures the state gets paid. Works quite well compared to other states that allow a lot of games with deductions. And yes, I am quite aware of the AMT. I was suggesting something only for top tier earners, not the AMT we have now (assuming deductions still exist).
And a true flat tax would have no deductions. That would never happen, which is why I alluded to a modified tax system.
I'd prefer a tiered income tax with no deductions (although it would end my career). The government could extrapolate what a tier of income normally pays with average deductions. So if a $280k-$350k earner pays on average an effective federal tax rate of 20% (estimate), then that is your tax rate. No deductions, no games. Make a tier of no deduction rates for everyone except low earners. Simplifies the system, and removes deductions. And it would also make people in states with high income taxes get motivated to lower their taxes. As it is now, with state deductions, states with high taxes keep more money at the expense of the federal government.
If business tax reform isn't included in the mix, then rich people will always find ways to game the system. And if the system allows for it, they should. Take advantage of the rules for the game board in front of you.