Sketch out what you want the back to look like. Get a rough idea what size you want the borders to be. You need to figure out if you can do the center in one piece or piece it to a larger size. Once you've got the center figured out, you can add borders to get it to the size you want it. I'd make it about 6" larger both length & width than the top's overall size. Mark the center length and width of both the top and bottom. You can align the centers when you sandwich the quilt.
Hope this helps and not confuses you. I might be able to draw it out later, but have to go - company's coming.