I turn my mat to the blank side, then square up the edge i plan to cut from. I take a ruler and lay it somewhere near where i think I will be making the cut, Then i take a smaller ruler, lay it perpendicular to the other ruler on the fabric, and measure over from the cut edge to the bigger ruler to get the exact measurement. check the measurements up and down the edge ("measure twice, cut once") and cut along the big ruller. Don't know if that makes sense or not, but it is a lot more accurate than using the line on the mat. Good luck!