How much did you spend over the course of a year? Simple -- take your income from all sources and subtract your net savings, multiply that by the NRST (tax-inclusive) rate, and there's your total taxes paid.
OK, that sounds a lot like filling out a 1040, with schedules A, B, & C: ... take your income from all sources ...OK, you've got your last paystub, or your W2 (some form of which will likely still exist for SS/MC wage reporting.) Then you've got your investment income, you know the Schedule B stuff, then you've got your capital gains (Schedule C), then you've got your rental income, then you've got your gambling income, then you've got your "other" income. I've probably forgotten a few, but today, I get statements ... probably still will under the FairTax.
That "simple" statement of "take your income from all sources" is no less complex under the FairTax than it us under the Income Tax.
Next:
... Now comes the hard part. How much did I spend on non-taxable purchases ... let's see where did I put those receipts ...oh, yeah... I didn't GET many receipts for my not taxable purchases. I got the one for tuition, and the E-Bay stuff ... oh I can probably look at my paypal account and my credit card statements to help figure that out; but then there are the garage sales and swap meets ... oh never mind. Hey you know, I NEVER had to track THIS much detail before just to figure out how much tax I paid!
With the income tax system, due to its complexity it becomes difficult to the point of impossible for ANYONE to predict a least-tax path for the upcoming tax period (year) and this is shown by the many instances of submitting a given set of parameters to different tax pros (including some in the IRS) and finding there is not only no agreement among many of them (actually almost none) but that the "paths" they independently devise vary widely in results of taxation.
With the FairTax, a taxpayer has the very great benefit in the immediate time period of knowing on each and every taxable purchase what "his government" costs him on his loaf of bread, bunch of bananas, or badass BMW - and that is far more immediately meaningful and useful to his decision-making process that finding out 6 months or more later that he paid x% in tax on that item (and cannot then decide to not buy it for that reason).
Trying to pose that one's calculating "all his taxes paid" under the income tax (or any other for that matter) is meaningful in any useful way is certainly not the case and most do not do that anyway since it is not possible even under the income tax due in part to the component of hidden taxes.