I always make a template when I do these. I tape a few pieces of paper together for it. I measure from top to bottom and find out what it is divisible by then take a compass and draw out the curves.
One thing I've found that helps me is that the paper should be as wide as your border for the visual. Depending on the width of the border and the impact that you want is how deep you go into the curve. Scallops can give a huge impact and more of them, IMO, is not always the best way to go.