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."