wheel measuring help

[lawrenq]

I'm looking for tips on how to measure for the correct backspace for my Chevelle. The car is mini-tubbed using the abc performance kit and the rear is narrowed 5 inches. I'm currently running a 313/35/20 on a 20x10 with a 5.5 inch backspace with a 2 inch adapter. If I'm figuring this right this means on a 20x10 wheel I'd need a 3.5 backspace to mount the wheel without adapters. I have the wheels mounted as close to the fender as possible. So if I'm planning on running a 20x12 wheel does 5.5 inches of backspacing sound right? I ordered a wheel fit tool but ordered the wrong one for my bolt pattern so I'm debating on measuring for wheels a different way. Here is a pic of the car.