This backspacing doesn't sound right!!!

This can be done with the tire on?



how would you get the rim flat on a board or anything else when the tire bulges out farther than the wheel ?? wouldn't the tire always hit the ground first. I'm sure there are ways to fan dangle it. But with such an investment hinging on good exact measurements wouldn't it be worth the time and few dollars to pull the wheel off and take it to ur local place to dismount the tire and get good accurate measurements off just the wheel.