I make my initial squares one inch larger than the "finished" unit size I want. This gives me 1/8" to cut away. If you still have trouble, add 1 1/4" instead. Press carefully with sizing or starch, then as NativeTexan says, use your 45* mark on your ruler. I do it exactly as she says and get perfect HSTs every time. Once you have mastered the knack, huge numbers of tricky blocks and patterns are suddenly easy. This skill is one of the important basics in lots of quilting and gives one greast joy once mastered.