Can you piece the back? Because you need a little wiggle room and it's better to have more around the edge than "just enough".
If you don't have a taught back you may end up with folds/puckers. Sometimes it just happens, but if it's smooth to begin with you're better off.
Sewing from the middle out helps for the back to "ease out" and flatten, but sewing can only do so much, kwim?