I, too, would double check the measurements of your ruler vs your mat. If they match up, just use the cutting lines on your mat. If they don't match, then it would be best to continue to use the cutting lines on your mat for the additional cuts. It sounds like it's a fairly simple pattern so you shouldn't have too many odd-ball (eighth inch) measurements to work with and the mat should be fine. As long as you are consistent in your measuring implements throughout a project it really doesn't matter which way you go. IMO.