RE: what fin size for a 6.8m on a S type104
Dolf, we have been discussing this before and I can see your point: I promised once to measure my fins and start to take notes of what sizes I used for different sails, but haven't gotten around to do it. In any case I'm sort of with Ian here from personal experience with wave and crossover boards. My main argument why I think (bi-) linear formula (even with a "personal coefficiant) is to simply is that in by definition locks the fin size to the sail size. I tend to look at sail-board-fin as a kind of holy trinity and I reguarly trim the feel (and performance) od a certain board-sail combo by changing fins (both fin type but also fin size). If we accept your formula as a basis, anyone who want to take full advantage of a boards "crossover appearance" will still have to have a set of coefficients for different types of sailing and then we're allready on our way into matrix land.
Actually, while I'm a fairly competent sailor on wave baords, I'm totally new to slalom boards and even with my new iSonic 101 and (my only) freeride sail of 7.5 sqm, I've so far have had equally fun experineces with the Slalom pro 40 and 34 cm fins. The choice for me depends on if I want to go for max downwind speed, and struggle a bit more upwind or just enjoy the (for me) fantastic upwind capability of the 40cm fin but loose a tad of top end speed maybe. I don't think this is an elitist way of thinking from my side, I think even many "casual users" wants to have the chance to use their boards to the fullest and fin choice is a part of this.
In any case, publishing the area of the Drake fins can't be a bad idea. Then people can do with this information what they want.