Ouch. Over all the years I worked and made good money and filed my own income tax, I never paid more than 17% real tax. Real tax is 100 * (what you paid Uncle/what you made). That calculation takes into account all the wonderful deductions that I worked so hard to take advantage of.
Perry offers a 20% sorta flat tax ( he still has various deductions). Tax payers get to chose between current law, and the new 20% tax. That's easy, most of us will take existing law, tiresome as it may be 'cause it's cheaper, like 17% or less. The "rich" (any one paying more than 20% under current law) will take the new Perry tax and save money. This doesn't sound like much of a budget balancer to me.
Was it me, I'd scrap the existing income tax law, all of it. Declare it repealed completely. Then pass a brand new law that has just three tax rates, one for the poor, one for the middle class, and one for the wealthy. No deductions for anything, except charitable donations, in CASH, with receipts. Everyone pays something. No tax credits, no dependents, no mortgage interest, no state&local tax deduction, no married/single/head or household. you make it you pay tax on it.