What you describe will give you four quarter square triangles (QST) - with all bias edges!! The formula for sizes is usually add 7/8 inch to the desired finished block size for a HST and 1 1/4 inch for a QST. Personally, I always add 1 inch for a HST and 1 1/2 for a QST and then square them up to correct size after sewing. Do a Google search for HSTs and/or QSTs and you will find several links with instructions. The best way to get QSTs with straight of grain edges is to sew HSTs first, pair them up and sew in the opposite diagonal.