Most people can't tell you the answer to that question. Of those that would answer the question, most will give you their marginal rate. Anyone who actually knows his average tax rate will, I agree, give you the inclusive percentage.
But nobody calculates sales tax that way.
Nor did anyone say that they did. Please spend some time on the FairTax website to determine that for yourself instead of just guesing about it - or taking the misinformation put out by FairTax opponents who, after all, have their own ax to grind.