Which ever method you prefer is fine, as long as you continue to use that same method for all cutting. It is not unusual for a ruler or matt grid to be a hair different. While it may not seem important, you add all those hairs up and you get a wonky quilt. Matts may warp over time and prevent you from cutting accuratly, therefore, I do recommend using your ruler for all measurements. Slide ruler from opposite side of fabric to your cutting arm. Such as, rt. handed person would use left side of fabric and lt. handed person would use the rt. side. After you are sure your fabric is squared up, place your ruler so the size strip you are cutting meets the edge of the fabric. Hope you find this helpful.