I was taught that the reason my blcoks were scant was because of the bump with pressing the seams to the side. A quilter taught me a trick whihc I still use to day. I position the line on the Omniruler just so that the fabric is line up with the top edge of the line so I have a fraction more in my cutting and my blocks come out perfectly. You have to find what works best for you. Good Luck.