I don't do anythingn so elaborate to measure....I cut the middle square and lay my 12" ruler on the 45º line on the top of the square and measure on the left side. Add 1/2" and cut a square that size and then cut diagonally across. you need two squares cut diagonally and then sew to two sides, press and sew to the other sides and square it up making sure you have 1/4" at the points. Sure proof everytime....BTY, I have two boughten SQ in SQ rulers and never could figure them out. LOL