Steve are they meaning finished at 13 by? Most pillows are sewn with a little larger seam allowance as a general rule. Might try laying your fabric over the pillow and then either pinning the "seam" to see how it will fit...or basting it....that way too you can mark where you want the stitching to end and the seam to begin. Have you allowed for the shrinkage due to quilting as well? Can understand fully not wanting to cover stitches. :-)