I oversize my HSTs and then cut them down. So if I want a 3 1/2" HST, I take 2 4" squares, place them right sides together, draw a line from corner to corner on the back of one. Then I sew a scant 1/4" seam on either side of that line. Then I cut the square in-half by cutting along the drawn line. I iron the HSTs open. Now I have 2 4" HSTs. I use a quilt ruler that has a 45-degree line on it. I line that up with the seam in the HST and use it to cut the HST down to the size I want.
Hope this makes sense. It is a super easy way and has never failed me!
Barb