Some say 12" from the center drain to the rough wall, others say 12"
from the center drain to the finished wall. *Best to get your toilet
before plumbing/wall construction.

Its usually 12" from center drain to finished wall. You can measure
the actual toilet that you pick, but the problem with that is what if
a couple of years down the road you have to change the toilet, and it
does not fit. Some manufacturers say their toilet is 12", but in
reality they could leave an 1" or 2 space, like the Toto toilets for
example. I just installed a 12" Toto and there is about a 1" space
behind the toilet.