I was taught that cutting by the measurements on the mat was inaccurate. Due to the thickness of the lines, are you measuring by the outside of the line or the inside of the line? That was why my first quilts were hard to fit. My strips were not really equal. So I always use my rulers and I cut all pieces for a quilt with the same 6x24 ruler. I square up with a 12.5 ruler. This way all blocks are exactly the same. Even a 16th of an inch can make a difference.