Your problem probably has nothing to do with math or with your ability to quilt. If you are doing this from a book or from online instructions without ever having any sewing experience or any lessons, it may be the basics or learning how to cut fabric that you need. I know that I certainly did. also you may be ironing not pressing and this can stretch the fabric. See if there are any local beginner quilting classes or an experienced local quilter who can work with you. Good luck. Don't give up.