I use two rulers to square up my fabric but I find as I cut my strips, I get wonky again so have to trim the edge again. I also learned not to go by my cutting mat lines as its totally off. The 1st 12 inches are accurate but as the inches increase it becomes inaccurate so I keep a metal measuring tape nearby especially when I'm measuring for my borders. Just thought about this, if I measure the sides or middle of my quilt top for the border and measure the border with the same measurement tool (cutting table incremates) then it should be the same since I used the same tool........right???? Sounds logical.............and that's what bothers me...............lol!