I know an 8" rim is measured bead to bead not edge to edge.

I thought I was trying to help you with your question not the other way around.

I have no clue why your stuff isn't working and no one else has taken the time to try and help you also.

My calculations of a stock 7" Ralley Wheel and factory trim ring work perfectly so I'm at a loss.

(Like I said earlier in this post a factory 15x7 has 4 1/4 backspace, trim ring is 2 3/4. 4 1/4 + 2 3/4= 7")