When cutting a square into four triangles - QSTs - theoretically, add 1.25 inches to the finished size. (I would add 1-3/8 inches for a bit of wiggle room)
When cutting a square into two triangles - HSTs - theoretically, add 7/8 inch to the finished size. (I would add 1 inch for a bit of wiggle room)
It's easier for me to trim off a smidge than to try to work with skimpy square units.