I have to ask the Stupid Question here, Why don't you just buy spacers from -- wait for it -- the tire store. They live and breath this stuff.

You need spacers because the inside of the tire strikes the suspension somewhere. The tire store should be able to see how hard the tires rub, and then suggest a spacer to cure the problem.

One thing to be aware of is that if you have rubbing on the suspension and cure it with a spacer, then you might end up with rubbing on a fender. This is cured by moving the fenders out of the way -- rolling fenders -- or by selecting a smaller tire.