That seems to imply just on the numbers alone that the tax would need to be close to 100%
That obviously cannot be true. Taxation cannot be 100% of anyone's economy. So there's something wrong with the numbers. If you simply replace the existing system with an NST that is revenue-neutral (collects the same number of dollars) then we know for certain it cannot be 100% because it's not 100% today.
Note that the NST would actually be revenue POSITIVE if you collected the same number of tax dollars because it is so much more efficient than an income-based tax system. Or, you could lower the NST tax rate to compensate for the increased efficiency and STILL have the same bottom-line tax revenue with a lower tax rate. Efficiency is a huge, huge benefit of the NST.
My 17% figure comes from a detailed analysis a while back. I wish I still had the paperwork but have looked several times and cannot find it. But it was well detailed and I came way convinced that it was within range. Let's say 20% for some margin. That's the federal sales tax rate, to which state and local sales taxes would be added. Here that would total 26%. ALL other taxes are eliminated.
Increasing it much beyond 20% would punish poorer people who disproportionally spend money on goods compared to their income
No, the basics of life (food, shelter, medical) are exempt. The poor pay zero for the basics of life. As they, or anyone else, start climbing the economic ladder and have discretionary dollars to spend, only those discretionary dollars are taxed. In this way the truly poor bear no burden at all, and as you gain ground you participate commensurate with your proportion *above* poverty. Very fair, and automatically scales to remain fair.
As to the "rich": Yes, investment dollars aren't taxed. But investment dollars, by definition, are poured back into the economy which generates economic activity that "lifts all boats". When rich people indulge themselves and buy something that is for their personal use, those dollars are taxed just like everyone else's. Fancy car = more tax. Private plane = more tax. Et cetera.
you can eliminate the 200 million tax returns by having the taxation service work it out automatically from payroll
You're presuming everyone is an employee, and further presuming that all employees are "above the table" with proper reporting to the government. Neither of those things is true. On the wealthy end, lots of people have income that isn't payroll. On the poorer end lots of laborers, illegal immigrants, etc. are paid off-the-books with no paper trail at all. The NST completely eliminates dependency upon paperwork and reporting, and treats every single individual the same no matter rich or poor, legal or not, employee or self-employed, under or over the table - because
everyone spends money. Tax at the point of retail sale and you get maximum efficiency and maximum fairness with zero individual paperwork, and you still have progressive taxation where the poor pay less/zero and the rich pay more. No other revenue system delivers all of those benefits.