Here's what I would do. I would cut a piece of freezer paper (or other long paper such as waxed paper) to the length of the quilt and another to the width. I would fold the paper in half, then half again, then half again to evenly divide the length into equal sections. Once I had a section size I like, I would experiment with available saucers, cups, or whatever I could find around the house that is round to find a curved shape that fits well within the section size.
Alternatively you could use a compass, drawing a straight line down the middle of the paper (or maybe several lines if you don't want a full half-circle for the scallop) and use a compass to draw between the folded sections.
I haven't watched them, but there are several Youtube videos that show how to draft a scalloped border:
http://www.youtube.com/watch?v=vkt4GQ93z0w
http://www.youtube.com/watch?v=rIHhTxTpMQ0
Also found this website:
http://ankastreasures.wordpress.com/...allops-part-2/