You don't cut with the lines on the mat at all. All you do is position your old ruler as you are used to doing, then abut the Slidelock against it, remove the ruler and make your cut against the Slidelock.
I don't do it that way at all. I have my ruler in place to cut and put the Slidelock on top of the ruler and cut as normal using the ruler edge. The Slidelock is for safety when using a rotary cutter and ruler. That's why I bough it, for safety, not for a cutting edge. I have two sizes and use the longer one the most. The ruler does not move with the Slidelock on it.