When I was in College studying Home Ec. Our professor said to use scan quarter seam allowances on quilting. I couldn't figure it out for a long time. Then it hit me, when the seam is pressed, it takes up about 3 threads, and that equals a scan. Also, when you use lighter weight thread it will change the seams a little bit. I like to use 60 weight thread to 100 weight on heavier fabric for piecing, gives the correct size finished block. On most sewing machine, if you move the needle position one to two position to the right, you will get scan quarter seams.