RE: what fin size for a 6.8m on a S type104
I agree with Phil and think your formulae (not just for fin area, but for sail size also) are vague, and of limited practical value. Some examples from my experience, tried and tested, confirmed by other (non pro) sailors:
-kombat86 + 6.0 -formula gives fin area of 270sq.cm, while I've found I'm using that combo with max. 230sq.cm, more often with 203sq.cm
-kombat86 + 5.0 -formula gives 225sq.cm, while I use it with 203 or (often) less.
-now an interesting example: formula gives me 328sq.cm for a 7.3 sail, and I regularly use 320 (41cm slalom fin). Fine, right? But that's a cambered sail on a wide slalom with chunky, blocky tail, so hardly relevant for recreational use.
-sail size formula tells me I need an 10.4-11.4 (!!!) sail for 10-11 knots of wind. 1.34x85kgx10knots=11.39 (I usually plane on a 9.0 in that wind.) If we take light (but planing) 7-8knots winds for example, sail size results would be even more ridiculous.
Now I'm not "poking holes" in the formula using "extremes and exceptions", but giving real world examples for others to take into consideration. I'll say again, I'm far from expert, just your ordinary sailor well beyond tuning "elite" you refer to so often.
These things (sail and fin choice) can be a nightmare for the inexperienced sailors, but over-simplifications will not help. A little bit of experience and advice (and hit&miss, yes) will usually take care of the problem. You said yourself everybody can adjust the coefficient to their needs, so what's the point of a formula, when it's always different?
Let me add that I don't dismiss your efforts and I've found your contributions valuable before (mostly re. aero and sails/fins for it). But I can't help noticing you're taking issues with anyone who dare to question your findings, you almost demand of *board team to adopt them, and there is a certain bitter, condescending tone in your postings.