How do I figure wheel offset?

[Vegas T]

Hello again guys, its been along tome since I've posted but due to overtime of the job (City Center in Las Vegas) my project had to be put on hold for about 4 months, but I plan to pick it back up within the next 30 days. Does anyone have a diagram to figure wheel offset for the back tires? I am assuming if I use a level to identify the face of the hub to the first obstruction of the body and subtract maybe 2" that would be my back spacing but I want to be sure this is correct befor I order $2000.00 in wheels and tires. Any info would help. Will post pics in the lounge soon...

 

[Lee_in_KC]

Here is a diagram done by BlownT...

 

[KAA]

What I did was order 1 wheel with the back spacing that I thought I needed. Tried it on. Turns out I was right.But if not I only had to return 1 wheel. Always check the the fit of a wheel before you mount the tire. Once you mount the tire the wheels are yours.