Offset is measured from the centerline of the wheel to the hub mounting face.

Backspace is measured from the back edge of the rim lip to the hub mounting face.

Keep in mind that every wheel is actually one inch (give or take a little) wider than the measured rim width from measuring lip to lip. The measured rim width is from the outer edges of tire mounting bead inside the rim lips.

Your 8.5" front rim is 9.5" wide lip to lip, you need to be aware of this to get a backspace measurement. Forget about the 8.5" number for now. Divide that 9.5" by 2 and you get 4.75". If your 8.5" wheel had zero offset it would have 4.75" of backspacing, make sense?

Your 15mm of positive offset moves the hub mounting face outward from the centerline. Add that 15mm (.590") positive offset to the 4.75" measurement from the back edge of the rim to the wheel centerline and you have a wheel with 5.34" of backspacing.

It's fairly simple math. But you just need to wrap your head around the simple relationship between offset and backspacing to make sense of it.