RE: Cutting down 70 cm fin to 55 or 62 cm
So, if I use a smaller fin I would be significantly better off by using one made for the size rather than cut down from a longer size.
The X-186 is my smallest board and might be my smallest board for several more years. I use from 4.5 to 11.0 sails with it. My 10.0 and 11.0 sails are old enough to not have much twist built in. The 10.0 is from about 1992. I think the 11.0 is from about 1995-96. My 9.5 is a 2004 Ezzy Infinity.
I'm not asking about "best". I'm asking about "workable, good enough, better than some".
When the wind is strong enough that 5.5 to 8.5 sails will get my X-186 on a plane while reaching would I be faster at some point with a 55 or 62 cm fin than with the 70 cm fin? Wouldn't fins smaller than 70 cm give me a better balance with sails that are 7.5 or smaller?
Reaching is as much what formula boards do as jibing is what an Equipe does. No board is the "best" at both of those particular parts of windsurfing, but both of them can actually do both of those things pretty well. Mediocre performance in windsurfing is still windsurfing and thats pretty good.
Unfortunately I think I would not be happy with any 100 ltr board. The winds I deal with on tiny lakes in the middle of the US have too many holes. I decide which board to use by considering the windspeed of the holes. Sail size depends on how big a sail I think I can hold onto in the gusts.