Barb, sometimes it is the simple stuff that escapes us... great tip on starting the slit first! I already have my stack of squares with the shapes drawn, and there is always that "be on guard" moment as I snip now.
About the slits on the back, some people make huge slits side to side, I'm more into just a couple lines radiating into the problem areas. Some people recommend then going back over the slits and stitching them down, I don't like the thread caught into the interfacing that way, but in a complicated shape it could be the way to go, especially if you were then trimming out the excess fabric.