My plumb bob measurements were from the mounting flanges to the edge of the wheel well lip to give me my outside clearances. As for the backspacing measurements, I used a straightedge on the back of the wheels and measured to the mounting surface giving me 3.5" or -32mm offset. To get a similar look on the outside but with a 295/50/r15 tire (295/60/r15 would be a much taller tire) you would need a wheel with a -6mm offset. I hope this helps.
FWIW, I meant a 295/50, not 295/60.
I have to say, I'm totally stumped now.
3.5" back spacing is a common offset for a lot of wider old school rims.
A 8.5" wide rim will measure about 9.5" wide from the edge of the lip to the edge of the lip. You said your plumb bob flange to front measurement was 5.5" and your flange to back measurement was 5". I assumed that was tire bulge to tire bulge since those added together is 10.5" and a 245/60 like you said you had mounted on there has a section width (widest part of the tire bulge) of 9.64" and that can get a little wider or narrower depending on the wheel you mount that on. If that's from the wheel lip to mounting flange that would add up to a 9.5" wide tire with 5" BS. EITHER measurement works out to a 1/2" (-13mm) offset which is way different then the -32mm that you measured with a straight edge across the back - the 2 measurements are off of each other by .79" by offset and 1.5" by backspacing.
I'm not sure what to do to reconcile the difference, I'm tempted to say that one of your measurements is just wrong (or I'm misunderstanding what you wrote), and without more information I'd pick the plumb bob measurement since nothing about your combination should measure 10.5" (the wheel should measure about 9.5" and the tire bulge should measure about 9.6-9.7"). OTOH, knowing that 0 offset is common for the magnum 500's and we have a pretty good idea what they look like with tires in this size range, I honestly have a hard time believing the -32mm/3.5" BS measurement since that would put the tires >1-1/4" further out than the same tires on magnum 500's. Huh... maybe but that's a lot of difference...