Found this on a search.
"To determine what size to draw the square, add 1” to the finished size of the triangle (if your triangle finished size needs to be 2”, draw a 3” square). This square size allows for the ¼” seam allowance.
I prefer to add 1" to the finished size of the half square triangle even though many quilters will tell you to add 7/8". For me, the even 1" is just easier to cut. "
That sounds right to me. If the pattern calls for a 3 7/8" square and the Go! Cutter does a 3" finished triangle...
Guess I'll just have to do a test. Does it really matter a whole lot if ALL of the HSTs are the same size? There are some 9 1/2" square solid pieces in the pattern but I can figure out if it needs to be a bit larger or smaller. There is no other shape but the HSTs in the whole pattern except for these larger background pieces.