I read the shimming 101 last year and it confused me.
I always break in a Go motor with stock shimming, no break in shim. After the motor loosens up a bit, a couple race weekends or a gallon on it I take out one brass shim. This leaves me with one aluminum and one brass shim. This is for 30% fuel, for 20% i think you could run one alum shim fine.

Rex