First of all, like everyone else on the board has said, there is no right or wrong way to do binding. I just heard from a friend who is a beginner quilter tell me that Nancy Zimmerman uses Heat n Bond to attach the binding before sewing. Never hear THAT before and I have have been quilting over 25 years. Many quilter who totally stitch the binding on by machine will start by stitching the binding to the back and then it is easier to make sure that the binding is caught with stitching by machine on the top. I would prefer to see a decorative design instead of attempting a straight stitch but that is just my opinion.
I would NOT change the 13th quilt because that would be a waste of time and effort. Just finish it as you have done with the previous 12 quilts and then on 14th quilt, try the method that most quilters use when hand stitching binding to the back.
There is NEVER only ONE WAY to do anything in quilting. Just have fun and enjoy the process of making a quilt.