Hi, when I made any HST's I always round up to the next number, in this case I'd cut 4" squares. Mark diagional, sew on either side of the line with a skinny 1/4", cut, press and then square up to size, in this case it would be 3 1/2". This way you are sure you HST is going to be the right size and the rest of the block will go together so much easier. You do the same thing with QST, round it up so when you get it made you can square it up to the correct size. :D Hope this helps and happy quilting. Jan