Since you are oversizing your HSTs and then trimming down to size, I recommend that you get the Quilt In A Day Triangle Square Up Ruler. It comes in 6-1/2" and 9" sizes, and you can find it t JoAnn's. The basic advantage to it for me is that after you have sewn and cut your squares in half, you trim BEFORE pressing open, so you only have to trim two sides.
https://www.quiltinaday.com/shoponline/rulers