For SCHRS... Simon and or his successor is the final answer.

But... the measurement of a boat is a measurement. If an ISAF certified person holds the tape measure.... they can give you an ISAF certified measurement form... good world wide for THAT BOAT.... AND the numbers are entered into the SCHRS system as certified.

If the numbers are submitted by some one else (third party)... you get the same rating but not the good house keeping seal of approval. SCHRS keeps your rating for your boat but doesn't certify it.

So, the majority of regatta OA's will use the SCHRS rating table as their standard. If some one shows up with a measurement form for their boat that is stamped ISAF certified... they use that rating for their personal modifications to their formerly class legal boat ... Otherwise they use the table ratings. If the measurement form is not certified... No good.... As always,... if they are cheating on equipment... they will be found out in a protest.

You could relax the standard as well.
Your OA could accept any third party rating certificate at their discretion.

At the extreme end... some events may say... We will measure crew weight and use that in the formula for a regatta specific rating for that boat and crew.... (PIA... but somebody might want to do it.)

At any rate... its the same formula and the same metric rule of the same boat parameters....

That's my read.