It depends on how deep your scallops are. I've done two scalloped bindings.
The first one had very deep scallops, so I measured the length & width, then added half the length again to the binding. I also made sure I had a couple extra binding strips just in case.
The second quilt I did had fairly shallow scallops. I measured length & width, cut one extra binding strip just in case, but I didn't need it, I had enough to go around.