converting offset to Back space

[Slow Ride]

I have a line on some wheels for my '77 Pontiac Ventura and want to make sure I'm calculating the Back space correct. They have 12mm of positive offset on a 8.5" wheel. It that 4.722" back space?

Also since I am having a hard time finding fitment info on 4th gen x-bodies. Would a this wheel with a 255/35R20 Falken tire fit OK?

I actually want 18's, but I can get these for $700.

Picture...