: Where does he get the 8.5' measurement? I don't see it listed
: anywhere in the table of offsets, nor anywhere else on the page.

The problem, like Dave mentioned, is that every boat by every builder will be a little different. I am in the process of preparing to build my first kayak(Sea Tour 15-R folder) and I had the same concern as you about some of the unexplained values in the plan, including this one. I plan on just seeing how the tubing looks once I get it all snapped into the cross sections and bungied for the first time. After that, I will probably just wing it with what looks closest to the profile drawing on the offsets picture. :)