I don't ever do 7/8. It's just too much work to measure that perfectly. I sew with 2 squares 1 inch larger than I want the finished HST to be. Sew 1/4 from the diagonal on each side. Cut apart to have 2 HST. Trim to perfection.
There are other ways that work well, too. I've had pretty good luck with the Easy Angle ruler, but still go back to the old favorite 2 square method.