When doing HSTs I always add an inch instead of 7/8 - then if my 1/4" isn't exact, I can trim them down to the right size, it sure has helped me have more accurate blocks. Even Eleanor Burns adds a bit extra and trims down and you're not wasting a whole lot of fabric. I'm not sure but check MSQC, I think Jenny has an easy way to make HSTs by placing 2 fabrics together RST sewing 1/4" all the way around, cutting across in each direction and there you go. It might be worth looking at some of her tutorials for easier ways to cut fabrics for quilts.