71 Demon 15 x 10 Backspace Question

Looks like 295/50x15 section width is supposed to be 11.6" (also saw a place that said a 295/55x15 was 12.2, not sure about that) and remember, your 15x10 wheel is really 11" wide, but backspace is measured from inside edge, so your 5.5 backspace mounts in the exact middle of the wheel.
Your sidewall bulge looks to be slightly more than 3/4" on each side. 1.6"divided by two.