How to calculate tire size?

mvh said
If the backspace is too small, the rim will sit farther out than stock, which is likely to cause clearance problems on the lower front corner of the opening

This is what I meant to ask, with all my babbling. I noticed the smaller backspaced Cragars were closer to the front of the lower fender.

I wish the Keystones were in better shape, I'm not sure if they are unsafe, no visible cracks.
As far as the tires go I would never drive on those.

I went to 15s in the rear, SBP, going to a taller gear and got the rims at a great price.

Keystones look about as wide as the chrome wheels I have, probably the same offset. I would blast and spray paint, they don't need to be shiny chrome.
Need a center cap.