I check mine (lap quilt sized) this way. Lay it on the carpet. I tie a piece of yarn to a safety pin, pin it to the carpet just above the quilt on the longest side and bring the yarn down just past the quilt and attach the yarn to another safety pin to the carpet. (on hard floors you can securely tape it down) Lay my longest ruler perpendicular to the first string of yarn to get a true 90 degree angle and run the yarn the same way across the bottom of the quilt. Now I adjust the quilt to these two strings. I can tell where I am off and I mark it with a ultra thin permanent marker along the yarn lines. I flip the quilt around and do the other two sides. Then I can take my mat to the quilt on the floor to trim or take the quilt to my mat.