I tried to link but I believe you add 7/8" to your square then you will cut on the diagonal. Could try a piece of paper then fold back the center then sew 1/4" seam allowance from center line down both sides. Or use a scrap of fabric you don' like. So your square of fabric would be 11 3/8''. Could not get measurements and thought I had a chart bookmarked but can't find it.
Sorry!