I agree that there are too many factors in this to make calculation worth the effort.
Hugh's value of thrust seems to be in agreement from what I have measured on guy wire loading, I found it to be a bit less, but near enough.
On the machine that drove me to distraction when the offset was too small, I did some tests after I got it furling and found that the seeking force was over 1/2 of the thrust so in my case perhaps 50% rather than 70%.
It seems to me that if the offset is on the small side then the seeking force holds it into the wind until the prop is at about 45 deg to the wind and then it looses seeking force and the thrust takes full control. The result is as Hugh described for the Bergey, it is virtually bistable and the output drops rapidly as it furls beyond 45 deg.
If you make the offset large then the power stays much more constant but you then have a much bigger issue with it running at an angle to the wind below furling. ( It does that a lot more than most realise anyway).
From reading comments here for years I also believe the load matching is a big factor and what happens for mine with the things running clear of stall doesn't seem to tie up with experiences here.
My only conclusion is that you need plenty of offset ( the 1/2" per foot looks marginal for this stalled operating) and you need to start with a nice light tail.
Never convince yourself that it is furling until you see the tail round to at least 60deg, otherwise you might get a rude awakening in the first real storm.
Maths has its uses but this case is more of an art than a science as far as I can tell.
Flux