I believe most times you add 7/8" to the starting square but I add a full inch and then trim my HST to the correct size.
So for a 4 1/2" HST square I would start with a 5 1/2" square, draw the diagonal, sew 1/4" each side of line. Cut, press and square-up to 4 1/2".
Do other agree with this?
kathyd