I always use Fons and Porter tool where you put the point of the tool at the corner, and draw your line on either side to the other corner, stitch those two sides on the line and cut down the middle. That is usually a 3-7/8" square and when it is cut, it is a 3-1/2" square . When the instructions have it at 3-1/2, I draw my line down the middle and end up wasting 1/2 the 1/2 square. I just follow the directions, but I still love making the 1/2 square triangles! Edie