I do have the quilt all quilted first. Then I take freezer paper strips - one for length and one for width - and fold repeatedly til I get what I want. This way I can get the ends to merge best. Or, if you like math, check for the commonality in the widths and lengths: divisible by 4 or 5 or 6. and work with that. Then remeber to have the ends at the halfway mark for the scallops so you can merge the two sides easily.