I just finished a quilt top (Tula Pink's Pining for you) that has snowballed corners, with great success. I tried the CG ruler and was not satisfied. Instead I drew my diagonal sew lines on the back of the large focus fabric, not on the little patches that can easily shift. This way I know exactly were the corners will touch. I have also taken to using this method for flying geese. No more wobbly geese for me.