Check out the following spreadsheet with class sail sizes:
J/30 Class Sail Dimensions Use the LP/J ratio for % size
Here are the dimensions on a new #3 as measured when I accepted it from Quantum Sails last spring:
LP 3594mm = 11' 9.5" (spec 3550 - 3658mm)
Luff length 9955mm = 32' 7.9" (spec 9982mm max)
Using J=3505mm (11' 6") results in LP/J as measured 102.5%
LP/J Min and Max % ratios 101.3% to 104.4%
[This message has been edited by Rhapsody #348 (edited 12-07-2008).]