I think a 2nd straight border about 2.5 inches finished (deep reds from your quilt colors), then a dark (to match your color scheme) for the scallop would look great!
I draw right on the good side of the fabric with chalk :)
I basically put a fat straight border on. Then I mark corners with a dinnerplate or my special scallop ruler to a "fullness" that I feel looks good.
Then I find the dead center of all 4 sides, and mark it with a stripe of scotch tape, then I visually (with special humpy scallop ruler) see about how they'll meet going from corner to center mark... choose a size and go for it.
I boo-boo'd one quilt, and got a point in the center when doing soft scallops (like a hilly road)... but I ended up liking it, it made an interesting focal point on the drop sides when I dressed the bed :)