I am not one to promote gizmos, but I have one of those lipped rulers that has become a favorite. It aligns perfectly with the squared edge of the mat, and STAYS PUT. If I'm not mistaken, the brand is Olippa, and cost is comparable to other rulers I own. I still use my old standard 6x24" for anything not requiring square cuts, but for 90 degree perfection, this is my tool of choice.