You haven't read Cain's plan.
You are right, I haven’t. If he is abolishing the FICA tax and rolling it into the Federal Income Tax then he is at least being honest, since FICA is an income tax.
His sales tax rate is too high on a national level. Most states without an income tax have 9-10% sales tax rate. If you have a federal income tax rate of nine percent the national sales tax needs to be around 2-3% if its sole purpose is to make sure that everyone pays some federal tax. Which is the only reason I can think of to have a national sales tax.
Remember the problem with the federal government is not tax revenue. It is spending and as Marc Rubio put it so well, we don’t have too few taxes we have too few taxpayers. Not sure if the 999 plan will do much for economic growth. But it does take the discussion in the right direction. So good for Cain on that score.