Let's say the tax rate is 7.75% (it varies by location in that state as in some others). The tax base presently is on a very restricted range of things with many exemptions, exceptions, exclusions, and special situations. It is nowhere near the entire consumption base since only tangible things are involved for one thing.
Conforming the sales tax to the FairTax would actually easily more than double the tax base since now services would be taxable as well and services typically are more than half of the dollar amount of consumption.
In addition, all exemptions, etc. (and there are thousands) would disappear raising the tax base further.
It is easy to see, then, that the tax base would about triple (or perhaps be even larger) which would bring the required sales tax rate into the 2% to 3% range.
It's pretty straightforward, actually.
I'll assume you have a basis for that, and accept it. That could bring it down from 7.75% to about 5.2% (if the politicians don't decide to grab some extra money, as they're wont to do). The rest is just unquantifed, unsubstantiated hand-waving.