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...
http://memimage.cardomain.net/member_images/1/web/2425000-2425999/2425608_88_full.jpg

[DriverzInc]

That would be a 5.25 BS. The 8.5 will measure 9.5 overall. Divide that by 2 to get the 0mm offset which is 4.75. 12mm is half an inch, so add that to the 4.75".
255 is a good choice tire for a 8.5" wheel.