Pam, what a cute quilt back. If I were doing the borders, I'd square off the last triangles right at the corner and use a square block the size of the border. Maybe make them each a different color or a square-in-a-square block. As to centering the backing, it will take some markings (center top, bottom, sides, very center of the quilt, etc) to use as reference marks to line everything up. Good luck--love that layout you used.