I was looking to change rims for performance enhancements as well as to accomidate brake modifications. So I pieced together all the numbers you need to be concerned with for rims. The formulas you will need as well to confirm the numbers.
Wheel Total Width ÷ 2 + offset = Back Spacing
Wheel Total Width ÷ 2 - offset = Front Spacing
Rim Total Width = Back Spacing + Front Spacing