I find my HSTs are more accurate if I "round up". I would cut 4" squares, put them right sides together, draw the center line and sew 1/4 inch away on each side of the line. I cut them apart, press, and then trim to the correct size (whatever they were supposed to finish at in the first place). I just finished four blocks with 72 HSTs per block (I'm cross-eyed!!) and doing them larger and trimming they all fit together perfectly. Sure there's a small amount of wasted fabric, but that's worth it to me if my HSTs are accurate. Did this make sense? I know what I do but sometimes can't explain it well-sorry if I confused you more.