Measuring for the correct wheel size/backspacing/offset?

[HotRodKush]

In my quest for new wheels for my Corvair, I quickly realized that nobody makes a bolt-in part for that car. That means that I need to check fitment, backspacing, etc. on my own BEFORE purchasing a set of wheels to make sure they'll fit. Is there a good guide out there somewhere that outlines the process for calculating the proper wheel fitment? I'd love to pick up a set of black steely Cragars, I just need to make sure I get the right measurements before buying them.