Reefing is very much a matter of crew comfort, as her handling does not change much as the wind and sea increase - the change in handling is much less than in a modern dinghy shaped boat.
Consequently, I have sometimes carried on too longbefore reefimg when sailing downwind. When the boom is dipping in the water on the lee roll it is definitely time to reef!
I prefer to run the boat off to change the jib or reef the staysail. Makes the job much nicer.
With old style roller reefing, and gaff rig, the mainsail can be reefed very quickly on any point of sailing.
All this stuff about upwind/downwind, type of boat, direction of wind wrt tide etc, is totally irrelevent, IMO.
The answer is much simpler - ie when SWMBO says so ! Her legs are too short to reach the lee side of the cockpit, so bracing positions become important. If there is too much tippy-tippy, I get told about it and can expect an interesting meal in the evening - hot tongue and cold shoulder !!
Forget all the crap about wind speed, realative angles etc. - blimey get a few yotties going and its Barnes Wallis hour !!!
I always remember what my First Captain said when I finally stood a Bridge Watch on my own .... If you think you should call the Master - You should already have dialed the number ! - better to call for nothing, than to not call for everything .....
Never fogot it - and I apply it to all aspects of my boating ..... reefing - which funny enough I don't do often - is done before I HAVE to .... different winds / different sea states, different circumstances - mean no set parameters ....
I don't have a wind speed guage in my current boat, but don't miss it for reefing as the angle/weather helm/lee helm gives me
some substantial clues about sail balance and reefing. I'd guess my boat, GK24, probably needs first reef between 12-15knots depending on lots of things, not least of which is how many people are on board, and where they are sitting.
The only thing I really miss is being able to say "Oh it's only blowing blah-blah knots, we've seen worse so we must be ok today". I don't race otherwise I'd be a bit more interested.