rim backspacing

My Scamp has Welds 15X10 with 8.5 backspace. I have 29X10 goodyear slicks. It is minitubbed even with the frame rail with stock axle housing and LBP Moser axles with B body brakes which widen the drum to drum easurement about 3/4 of an inch.The lip of the wheel opening was shortened slightly for sidewall clearence but the real close area is the front of the wheel opening. When doing a minitubb and relocation move the whole axle rearward and inch or so, it gives you room for taller tires. I have about 1/2 between tire and front of wheel opening with mine moved back. I relocated and minitubbed,then hung a plumbob and plotted the measurements on the floor. I wanted Centerlines but went with Welds because they (Summit) stocked the 8.5 BS.