I first straighten the edge (cut off the selvedge), then flip the piece over and measure from the left with my 6"x24" ruler and cut that way. The rulers are much more accurate than the markings on the mats. Also, put the outside edge of the line on the ruler even with the outside edge of the fabric so that the printed line is completely on the fabric. Just be sure you cut all your pieces the same way so that everything fits. If you measure a little differently than this, it's OK as long as you are consistent.