The way I've been thinking, unregistered is right. When completely powered up on a given board and sail, the heavier sailor will have the potential to go faster than the lighter sailor of the same ability who is also completely powered. (The bigger sailor needs more wind.) If speed was all that the fin 'sensed', then the heavier sailor may get 'overfinned' sooner.
I always thought it was kinda funny relating required fin size to sail size. There seem to be so many factors, so I guess you have to start somewhere. But fins are pretty stupid. They don't know what size sail is on the board. What do they 'feel'? For sure, the lift they generate is proportional to speed through the water. But, don't they also feel a sideways force. A bigger sailor will generate more of a sideways force than a smaller sailor, again, both being fully powered.
Then again, that bigger sailor being out on the rail probably needs more lift. Who knows? I think I'll just 'guess' on my first fin by getting the cheapest used weed fin I can find, and working from there.
Sergio, what are some of the 'softer' weed fins you know of?