So just to be annoying, if the charge is 6.00 at taco bell, the sales tax would be 6.00 - (6.00/(1+sales tax rate))

(There's gotta be a way to simplify that equation but it's not occurring to my tired brain tonight).

More pragmatically, the IRS sales tax table is based on your income. If you're a mustachian in the accumulation phase and spending a lot less than you earn, I'd guess the estimated sales tax paid is actually quite a bit more than you actually paid, so it'd be better *not* to track actual taxes paid.