When I get my quilts back from my LA quilter, the whole quilt is quilted and that's when I square up my quilt. My LQS owner taught me to do it then. I don't know if everyone else does it that way, but it works for me. So, maybe wait until you quilt your borders, then use your 12 1/2" ruler and square up your quilt. After you quilt your borders, they will be a different size all together.