I knew there was a formula for that; I just could not remember what it was.
I've been drawing everything out on graph paper. My fail-safe is to cut the block 2" larger than I think I need after drawing it out, cut on the diagonal(s), fold on the centers of pieces to be matched, sew and trim. So far, it has worked, but the math is better to do before cutting, IMHO.
So thanks, Pyth!