I birth most of my table runners, but I use a different method. I cut my backing about an inch longer than needed, then split it down the middle. I seam the two pieces with a large stitch and 1/2" seam, then sew the sandwich together sewing all the way around (no opening). I open the seam in the center of the backing, birth the item, then handstitch the opening closed. I can take up any extra backing fabric by pulling it into the center. If I'm really careful with my stitches I can still use the runner reversed.
I love your runner. I think I may make one similar. Thanks for the inspiration.