The sailor is responsible for his rating.... If the RC made a mistake... you don't get to keep the trophy.... you are supposed to report it.... and correct the scoresheet.
Doesn't matter if its portsmouth, PHRF SCHRS etc. etc.

So, you make changes to your boat... you are responsible for correcting the rating if its warranted. You have to know your proper rating for the modifications you made.

Same principle in SCHRS.... the sailor is responsible to know his proper rating.

For your example... There are lots of options... He could get the measurement guidelines.... measure his own sail and calculate his rating. That is his measurement rating!

If the OA... says... Harry, you are blind... you can't read the rule... get somebody else to do it... Presto he has a third party rating.

The OA could say... Harry.... just get the numbers from the sailmaker.... the sails may have shrunk over time... but hey... its measured by a pro... if it matters to you get them remeasured. . So, he calls his sailmaker.... gets the numbers for his his non stock sails.... opens the calculator and crunches a new rating...

If he can't find the sailmaker... he can get any sailmaker to actually measure the sail using the SCHRS rules ... get a sail measurement certificate, crunch the numbers in the calculator and he has a measurement rating based on an even more authoritative measurement.

If the OA requires it.... (big deal race.....) He can go to a US SAiling /World Sailing sanctioned measurer and get an Official SCHRS measurement certificate.

The sail foot is 150 cm...... It doesn't change based on who is measuring it... What is changing is the level of authority which reports the rating to the OA.

This is no different then Portsmouth... you were supposed to report all modifications made to the one design boat that Portsmouth wanted. Calculate your rating and register your boat using it's proper rating. It doesn't get you off the hook if the scorekeeper did it properly or if he made an error... it was always your responsibility to enter with the proper rating and check it.

Remember, in Portsmouth the adjustments table was not allowed for important races like Area Qualifier regattas because the adjustments were just best guesses.

The OA sets the rules and level of compliance... the sailor meets the standard or not.
Can you be lazy and half butt it... sure...
The good news is that your decision only effects your fleet and racers... The decision on ratings does not effect anyone outside your fleet. the data are not collected so ... YMMV.