You can use adding machine tape to measure and cut the scallops. Measure the exact length and fold the tape in half. Fold in half again and a third time--even a fourth and fifth, until you have folded it small enough to draw 1 or two scallops on the paper. Cut the strip out on the drawn scallop line. Unfold and you should have a line of scallops long enough for 1 side. Repeat for the shorter side. You can reuse the two strips to draw the scallops on the other two sides. Should one side be slightly longer than the other, stop, slide the strip forward just a smidge and draw the scallop and then repin. No one will be the wiser and the scallops will fit the border just fine.