Barbara, if you are making half square triangles (HST's) to fit with 3 1/2" squares, you need to have your squares for your HSTs to be 3 7/8" to begin with and sew 1/4" from the center diagonal line you mark on it. You should end up with 2 HSTs that measure 3 1/2"
I actually sew a scant 1/4" and works really well.
Hope this helps........if I didn't explain it enough, let me know.
Ellen