There is no formula. Just add 2 inchs for each size up. For 1 1/2 inch triangles use 4 inch blocks, for 2 1/2 in triangles use 6 inch blocks ect.
Of course since they dont make rulers that size I just use a ruler with the extra 1/2 inch. It wastes a little fabric but not much.
The triangle square up ruler is worth every penny it costs. After I show people how it works the go buy one.
So easy to just make one cut for perfect HSTs.