The absolute best ruler for squaring up HSTs is one called the Bloc Loc ruler at
www.blocloc.com. It has a little groove that your 1/4" seam sits in down the middle and nestles right up to your seam. It won't shift and makes squaring up so easy. Get the biggest one which is 6 1/2" and you can use it for smaller sizes too of course. I love, love, love this ruler!