Hmmm... I just googled paper pieced HSTs and I see what you mean slbram. Okay, take a piece of graph paper (or any paper for this test would work but the thinner the better because it will be easier to tear off) Draw your square being very very careful to be acurate. Now draw your diagonal line. You know that if you are piecing HST's you need two squares about 1 inch bigger than the final HST. So, cut those two squares and then cut them diagonally. Lay one on your paper, making sure that you have 1/4 inch or slightly less, over that diagonal line. Pin it in place. Now take the alternate coloured triangle, lay it over the first fabric, turn the whole piece over and stitch along the line. Open, finger press and check how you did. Is it more accurate?
I made a paper pieced double flying geese border for a quilt a few years ago. It was the only way to get it as accurate as it needed to be and even tho it was waaaaay more work than I expected, I am super happy with the result.