sail size X5 + 3 = fin size
so 4.0 sail..... multiply times 5 = 20 + 3 = 23cm fin
there are several "in the ball park" formulas for fin selection, since windsurfing is not a math type science , thats all these math type formulas will do, I noticed Roger stated
80KG , I have seen that 75KG is the average weight when referring to fin size, board liter float on..on Being 57KG (eat more cheeseburgers)
using my above posted formula I would look at 20-- 21cm .