Karen Kay Buckley has a DVD on borders and how to do the math. AliKat's method is very close, except she doesn't use freezer paper. I took her class last year at the Houston Quilt Festival and she gave us each a copy of the DVD. You can find it on her website, but I bet Amazon carries it, too.

I only ever did one quilt with a scalloped border, but I didn't use any special ruler, just used a dinner plate . I cut my fabric to the width I wanted, then I put tiny pieces of colored tape on the edge of the plate where I wanted to mark the same size for each scallop. I marked off my borders with a permanent pigma marker, but did not cut them at that time. I can't remember for sure, but I believe I marked the lengths first, then the widths, and mitered the corners to make them fit. When I took the quilt to my LA'er, she thanked me for leaving the extra on, since it was easier for her to quilt around the straight edge, rather than the scallps. After the quilting was finished, then I cut out the scallops and bound the quilt.