Exactly what Tartan said, coping lets you fudge in a bit when you are going from straight blocks to diagonal for one example. In your case, you might easiest be able to trim the red inner border to make things fit better, or you might need to add in an extra strip of white (which would also help stabilize that edge).
When I set quilts diagonally, I typically make the setting triangles extra large for a couple of reasons. One is I like to cut large and trim for accuracy, and sometimes it just all works better if you can "float" (where the blocks of the inner/bed part don't meet the border blocks) the quilt a bit with that extra fabric.
Edit: I love that book btw, I used the footprints for my own cat quilt.