Cut a 4 inch square from your left-over quilt top fabric. When your flannel comes, cut a 4 inch square from it. Wash the two squares and dry them. Trace a four inch square on paper and see how your washed and dryed squares measure up. If they both shrank at the same rate, then go ahead and quilt. If your flannel really shrinks alot more, maybe you will need to pre-wash it.