SewingPup and Dunster ... agree with what you have said, though I don't think Mackie noted whether the quilt will be long armed or domestic machine quilted or perhaps even tied?
Mackie ... another way some have handled similar situations is to use extra blocks from the front on the back to give some extra width. It would mean splitting the backing fabric to allow you to piece in the extra blocks and not necessarily down the middle. I usually go for about 2/3 and sometimes add extra blocks both horizontally and vertically. Or just work up a simple design of leftover pieces of the front fabrics for the backing.