I believe the formula for figuring the size of HSTs is to take the size and add 7/8", so for 3-1/2" HSTs, you'd start with a 4-3/8" square. However, since cutting anything in 1/8" increments is a pain, I do the same thing Maniac does, make the squares 4-1/2" and then trim. Eleanor Burns has a Triangle Square Up Ruler that makes trimming easy, because you trim the triangle before you press it open, so you only have to make two cuts.