I don't know but doubt that it is higher than 15%.For 2004 the FairTax base plus exports (I'm limiting it to that so that there is no squawking from the FairTaxers) was $11,582 billion. That is the total "price" of goods claimed to dropping after the FairTax is passed. The corporate income tax collected in 2004 was $190 billion, or 1.64% of "prices." The employer share of payroll taxes was $383 billion, or 3.31% of "prices." That's a total of 4.95%. Even if all of these taxes were incident on consumers (and virtually every economist believes they are not) that is no where near a 20-35% price drop.
The argument takes those numbers (or their equivalent) then cycles them through the economic chain thereby increasing the impact which is a valid procedure since each step along the chain must pay them.
I agree the numbers do not add up but neither does the economic theorizing which uses them. Such as the idea that income taxes are included in the cost of production.
And you are entirely correct that workers will have to take nominal pay cut for the plan to work. The chances of THAT happening is between zero and none.
The whole thing requires far more faith than I have and I suspect I am not alone in that.