I have been using El Burns triangle square up ruler for a long time. It is the best investment I ever made. Got it at Joanns with a coupon.
You only make one cut to square up any size up to 6 1/2 inches. Place the ruler on the stitch line before opening and line up with one edge and cut the other. Perfect HST every time.
I also use her method for making 8 HSTs at a time and whip them out very fast.