That is the point. Unless you total the tax, all year long, from every receipt, then adjust for you monthly prebate, you won't know EITHER the amount of tax you paid NOR your real (effective) tax rate.
You did not address that point.
I said nothing about knowing you're "rate" ... but that's all you focused on. What you say is actually insufficient to determine your true tax burden.
For all it's warts, at least I know how much I pay in Federal taxes: 80% to 95% of my total burden is printed, in plain sight on my paystubs and W2. 85% to 95% of it is finalized on my 1040. The other 5% to 15% is truly hidden ... but it's easy to figure out, and it's very small part of my overall burden.