Sounds like you got it figured out! You don’t need a whole bunch extra on a sit down machine, but you need some to hang onto when you are quilting near the edges for control. As you know, it also helps when you are layering your sandwich to have some wiggle room if you can’t see where your backing is to line it up. I’ve done some squeakers with charity quilts where the Quilter didn’t give me anymore than just enough backing and it all comes out just fine.