What vehicle is that (I'm guessing Acadia/Traverse? - looks exactly like my Outlook).
I don't like the way the belt is through the guide with the back on, did you try reclining/moving the seat forward/back with the back on the booster?
For using it as a NBB I'd slip the belt through the built in belt guide, it's the little plastic guide on the elastic that attached to the wall just below the seatbelt. That helps a lot when using NBB (I don't remember if my 9yo is still using it or not).
When I used our FR80 it was way too wide in that spot (oddly it worked better in the 3rd row, driver's side). I've removed my headrests for when we use boosters (not hard to do, just push down the fabric to access the metal clip that's holding them on). The Parkway was OK (newer ones would probably work better) and the Performance Booster is great, although it's wide and often needs to be scooted over so it can be buckled easier.