It is a matter of choice. There is no one real
formula for chosing a winch.

wrote

I do not think so, Andersen has a 40 winch why not recommend that for a
300 genua, if that is what is needed. In my opinion they calculate what
you need based on diffent algoritmens, and so who is right?

You are right, there is not a formula that every one agrees on.
(perhaps that is mostly a matter of marketing spin, there is perhaps a
tendency to recommend to small winches.) Anyhow here is one from Harken
site, that I thinks look good. Does anybody no of something better?

"Calculating Winch Power
Harken: WinchesTo calculate desirable sheet winch power, figure your
foretriangle area (the area bounded by the forestay, foredeck, and
mast); multiply it by 6 (or 29 if you figure your foretriangle area in
square meters), a number that accounts for friction and sail force; and
divide the answer by 35, your desired maximum input force in pounds
(or.15.9 kg). The answer you get will be the rated mechanical advantage
in second gear (low gear) of your optimally powered winch."