It doesn't really matter what the sailmaker's software says. The real question is what does the sail measure. The class rules need to state the procedure for measurement, and what the legal values are. If they don't, then the class rules are lacking.

For example, I sail in one class where the luff, leech, and foot of the sails are measured under a specified load. There are minimum and maximum allowable lengths, and maximum chord depths at the midpoints. As long as those values are met, the sail is legal - regardless of the actual area.

As others have noted, sails are pretty much always measured flat despite the fact that modern designs are 3-d. The area we measure will be different than the modeled surface area.

Also, when the bolt-rope (if any) is sewn in, it will be made short in order to allow for stretch. If the sail is measured without proper tension on the bolt-rope, the area will come out significantly smaller. As a sailmaker and Isotope class measurer, it took some time to work out assembly and measurement procedures such that the numbers before and after attaching the bolt-rope matched.

Also, realize that sail measurement is not an exact science. Measure the same sail twice and you'll wind up with small variations in each value. I figure that my margin of error is at least +/- 0.5%, so if the sails measure within 1 sqft of target (total), I call them legal.

Sure, it doesn't matter what the computer says. But if that's the basis for the sailmaker thinking he is right (comparing stock to square top differences) the issue needs to be looked at and the sails measured.

Most of the class rules I have read refer this back to the ISAF Measurement Instructions.

As for providing the measurements.... I think Prindle did this with the exception of the 18-2, and allowed for any sail maker to make sails... Like Hobie and NACRA; Prindle didn't provide the measurements 18-2 as again "legal" class sails were from them or a licenses/authorized sail maker..

Man jake I forgot about that 5.2! Diamond Dave has my old 5.2 and has spent a few years rehabbing it but is now on tour in Afghanistan for one year. he wants me to complete the rehab and sail the boat, or more or less get the boat sailing for his new bride.
It has a new Sq top, new jib(standard size) and spin. I think Whirlwind made them, hopefully I can check it out this summer and get it back on the water.
Those were good days on the 5.2, seems like forever ago, I guess it was 1990-94 for me. Boat is a 1976 model.