Generally long arm quilters want 4" extra on each side. So a 9" border on the back would be appropriate. But please, as bearisgray said, keep in mind that getting the back perfectly centered when longarming is *really* difficult, so you may end up with a 3" top border and a 5" bottom border when it's done, for example. I did this with one quilt (of my own) recently, and despite my best efforts I managed to mess up the alignment top to bottom.