J/30 Class Association Forums J/30 Forum One Design Class Rules Admin Error found in Class Rules for J Dimension

Just rereading the post above, and the ISAF Equipment Rules of Sailing that control measurement specify:
H.3.4 Unless otherwise specified, longitudinal measurements shall be taken parallel to the longitudinal major axis.

Class rules do not say to measure diagonally, so it appears there is no problem.

Edit: As always, official rules interpretations must be made by the Chief Measurer per J/30 Class Rules:

1.6 Official interpretations of these rules shall be made by the Chief Measurer of the J/30 Class Association who may consult with the Class Board of Governors or the Class President or the International Sailing Federation (ISAF). Fleet Measurers shall consult with the Chief Measurer when making interpretations of these rules at the local level. Interpretations of local rules shall be made by the Fleet Measurer who may consult with the local fleet organization.