I use the lines on my mat to square up the fabric, but I never "measure" fabric with those lines. I was taught to stay with one brand of ruler because even rulers can vary and certainly a mat would be even less accurate. I always use Ommigrid rulers only because that's what I have the most. I use other specialty rulers for angles, special projects, etc. I also use an Olipfa ruler as my straight edge with a rotary cutter because I like the stability I get from "hooking it" onto the edge of the mat. An additional aid, for me anyway, is the Gypsy gripper. My rulers seem to slip less when I use the gripper on the straight edge I am using as my cutting edge.