I have found that by making the HST's slightly larger, like even 1/8", I mark the diagonal line, sew on either side, press, then trim to the correct measurement. It seems that no matter how careful I am, I usually wind up trimming to the correct size, some more than others, but it works for me. I would rather take the time to trim after sewing than to have pieces the wrong size, even by just a little bit.