Do you buy a board because of the fin supplied. The supplied fin shoud give a reference point as to where you should start to look when adding new fins to your quiver. If you spin out on a given sail/board/fin then try a different fin, if you feel over finned, change again. There is usually someone with similar kit around that will let you try a fin of theirs. This kind of trial and error teaches far more than any formula could. Starboard alread show a suggested fin range for each board so that also provides guidelines. On top of that the fin manufacturers are very helpfull when choosing a fin so an email to them also helps to make an informed decision far better than any formula could.
I did sit and think about the formula thing for quite a while a couple of years ago but there are just too many variables to get even close to a working system. Even trying to simplify it gives far to vague a system.
It would be good if starboard would state whether the supplied fin is intended for the top,mid or bottom of a boards sail range and each fin name/size in the boards descriptive data to link directly to the relavent fin on the drake site.
p.s. a list of fin manufacturers can be found here http://www.windsurf.me.uk/Links/links.html