Wogboy
You ask for the wheel offset, then describe it as the distance from the wheel mounting face, to the back of the wheel. WRONG.
These are two TOTALLY DIFERENT specs.

The distance from the wheel mounting face to the back of the wheel is BACKSPACE.

Wheel offset is the diference in distance from the wheel mounting face to the imaginary centerline of the wheel. 10's & 20's has got it right.

The way to measure it is to simply measure the overall width of the rim, & divide this figure in half to get the centerline. Next, measure the backspace. If this number is greater than the first, or centerline figure, [it usually is] then subtract the smaller number from the greater & the result is the amount of "negative" offset.

If this backspace number is smaller than the centerline figure [rare] then deduct the smaller from the greater & the result is "positive" offset.

Please do not disseminate misinformation.