I didn't miter the corners on the one I did, either. I used Eleanor Burns' ruler for scallops and wavy lines, can't think of the exact name. It came with very helpful instructions and made it pretty easy for me without having to do any math for myself.
Also used the McCalls.com free video instructions for making the bias binding which is required for a scalloped border. It worked really well for me. I used a stripe for the binding and was so pleased how it turned out. It was fun. Good luck with yours!