Does the FairTax calculator say what your tax rate will be if you get laid off, perhaps because a taxpayer revolt has put your employer on the skids, and have to borrow money on your house to get by until you find another job? Not only do you have to pay the 30% tax on the bread and butter put on the table, but also the portion of the interest and loan service that is taxable. The decent into FairTax hell begins.
That bring up an interesting point about 'the effective fair tax rate'. If I am a retiree with very little income, but lots of savings, my 'effective fair tax rate' could easily be 3000%! The fairtax calculator assumes I can only spend what I make, not what I save. Another fraud of the fairytax calculator.
To answer your question, that's not up to the calculator but the one using it.
Also, interest isn't taxable under the FairTax.