I guess the point I'm trying to get to is if you have a reasonable expectation of wind speed and water conditions, you may be able to make a better choice of rigs on any given day. Ever been halfway down the run and thought (I should have rigged the so and so?).
If there was a given ratio that was readily available (along with some of the brilliant ideas on this website for fin selection based on sail size) it would make the choice of not only boards but fins for speed sailing on that particular run a lot easier, thereby removing the (I might go with this today, cos it worked yesterday factor).
The idea of getting everything to work together is paramount to having a ball, or having a short uncomfortable blast. (For me anyway) If you speak to someone who regularly does over 100ks in a day, they might say something like "I just felt comfortable so I kept going". So, that's what does it for me, the comfort factor. I'd rather have an enjoyable day with friends on the water, than trying to break my neck chasing numbers. To get all this right, it's important not to have to change rigs mid-session. (Unless you've got a rig bitch)

. Life's short, try to enjoy it more..