The NP V8 is one of the most powerful light wind sails around.
Not sure your "equation" is going to work with a sail this powerful
and a slalom oriented board like the S-Type 137.
Give it a try, if it spins out alot, look for a 52-56 cm slalom fin.
I've never been able to apply mathematical "equations" to windsurfing
Sometimes the formulas work out, often they do not.
Hope this helps,