In response to my own post...

I think I worked out a rough formula: Top and bottom of cover = measurement from one middle of insert width to the other (i.e., middle-of-one-side of insert to middle-of-opposite-side of insert). Measure 1/2 of width of insert away from each corner. Pinch together at that point and baste. When cover pieces are stitched together, the SA will eat 2" overall = proper insert:cover ratio.

I really hope this makes sense to me in the morning...