Hi Geoff,
The "stated length" is the depth below the bottom surface of your board.
As Wolfgang has described above the "diagonal length" is 1.414 x the depth of the fin
(for a 45 deg. Leading Edge angle).
This is the secant of 45 deg. (1.4142).
I believe that most fin producers classify their weed fins by the "stated length".
Hope this helps,
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