I suck at math

I spent one hour trying to draft an approximate pattern for my mom's skirt, and there's way too much math involved for me to like the process. At least, I think I have a good base.
So the design is settled, a jacket like the one on the left, only with a smaller, shawl collar, and the long, straight skirt, with knife pleats on each side. I was lucky enough to find a good approximation of the jacket pattern on http://www.marquise.de , but I have to draft the skirt pattern myself.