Hi macasterj,
There is a definite relationship - and especially with the HP series - with fin depth, and BOTH board length and surfer's weight.  The board's tail width and design also can play into the formula.  Our Spherical Revolver's tail is too wide and round to take an HP Fin, so I designed a special fin with a wider neck to reduce the fin flex.
The basic rule is a 9-0 board takes a 9.0 HP fin.  A 9-6 board a 9.6 HP fin, and a 10-0 board the 10.0 HP.  Larger boards to 10-6 also seem to work with the 10.0 HP.  The new Super HP fin, with it's added volume in the neck should fill in for the (larger) people who find the standard shape HP series too flexible.
If the surfer is riding a board larger than the recommended size, then the fin would need to be moved ahead of 8.5" from the tail.   And if they are heavier weight, move towards the tail.   All of these adjustments should be in 1/4" increments, as already suggested by "screwfoot49."
Also coming into play is the surfer's ability.  A better surfer, moves it towards the tail.  Surfers that don't turn with as much athority, set fin further forward.
It should be noted here, that riding any board outside of our recommended fin size/board size will not deliver the intended results.  This applies to most of the general public.  However, there is always some really good surfer who breaks all of my heretofore mentioned recommendations.  And that folks, is what makes this job not a job.

Last Edited By: Rich Harbour Aug 30 12 7:36 AM. Edited 2 times.