Wheel Backspace Makes My Head Hurt

[MAGGEE]

I’ve got a 1972 K10 with 2 inches of suspension lift. I’m ordering a set of wheels from Detroit Steel Wheel. 20x8 or 20x9 (more likely). Backspacing is custom. Up to 4.75 inches on the 8s and 6.00 on the 9s.

This is similar to the stance I want. Front and rear. Even with or just a bit outside the fenders.

Initially, I’ll be running a 34 inch tire with 11 inch tread width (275/65/20).

Calculating the proper backspacing is making my head hurt. Any suggestions? Should it be the same front and rear? Most of the backspacing info I’ve found for our trucks relates to dropped trucks seeking to keep the tires inside the fenders.

I need help.