Melissa...I know what you mean about HST...I think everyone can have a hard time with them...one thing about this is you do have some room for error....when you draw the diagonal seam allowances, if you take a smaller seam allowance at this stage, say 3/16th's, after you have made the HST you will have a little extra to square up...check out my pictures you will see that is what I have actually measured at is .5cm (my ruler is metric!!) :D