I used the EB scallop ruler. It takes all the math out and makes it simple.

I only ever did one scalloped edge. I made my border fabric a little wider, and marked my scallops, but left the edges straight, until after it was quilted. My LA'er thanked me profusely for doing that. After it was quilted, I cut the scallops out, and bound the quilt with bias binding. It took me a little longer to do, but the results were very pleasing to the eye, and the recipient just loved it.

MSQC has a template as their daily special today!! its a great deal. This is the one that I use

https://www.missouriquiltco.com/shop/dailydeal

I use EB scallop template too. It's goof proof.