First, try measuring from the "finished" wall, not from the baseboard, and see if that makes any difference in your calculations.
There is no standard for how much room should be behind the tank on a 10" or 12" rough-in toilet. It only means that the toilet will fit in that space for sure. In the Toto line, there are toilets (just a couple) that require the whole 12", which is very unusual. Most will fit on less. Typically, it's 11.25 or 11.5, but one will go on 10-7/8".
Use two wax rings (no funnel on bottom one, one funnel on top one to hold it in place) or an extra-thich (#10) wax ring.