The other thing to remember is that thread and fabric affect the seam. I know where my scant 1/4" inch usually is, but I always sew a trial block and sometimes I have to make an adjustment because the thread/fabric combo throws it off. so you need to verify at the beginning of each project. Cut three 2" strips, about 4-6" long as accurately as you can. Sew them togther and press. The center strip should be exactly 1.5".