I figured it at 12" just to be safe. The blocks will be 9" including seam allowance (the backing is cut 1" bigger than the unsewn blocks right?) so that gives me a .5" leeway. In the long run I'll be money ahead I guess, since I won't bother trying to machine quilt something that big on my machine so I'll save on longarm fees. Still that is a lot of fabric! I knew it would be more for the seams but I didn't know if it would be that much more.