I am also assuming the tha the measurments are for the rear shapers mark and not the acutual rear base of the fin? As they do seem rather far back, so maybe they are the fin itself?
Why not ask Bruce himself? I did on my last board, via quick email, and he replied within a couple of days. His email address, which is publicly available (so I’m not breaching any privacy by posting here), is mckeequattro@hotmail.com
there are so many ways to set up a quad. How wide is the tail, what shape is it, what wave type are you planning for (grovel or barrel?)… All these things matter.
Ps Chris p… You’ve probably just not ridden a good one yet!!!
Just look at the measurements as they relate to declining lengths of boards and then just calc it out. While his dimensions are a good starting point they aren’t the be all end all perfect dimensions for every board.
This past season I built a few quads and kind of split the difference from on-the-rail back fins to the McKee setup and that is working particularly well.