**TIP** On sewing down the back sashing.
Since the front sashing becomes 1/2" wide, if you adjust your needle (if your machine can do that) to just inside that 1/2" mark, your stitching will be inside the sashing. I'm still perfecting this myself and still wander off some time but, hey, as my quilt instructor always taught me....."Can you live with it?" If not, redo.
On a separate note, did I miss the post where you state what widths to cut the sashing strips? I cut mine 1" and 1-5/8".