As such, lets just agree that, class legal is class legal. If you could take your H16, F18, A-cat, P19, N6.0, etc. etc. etc. to a nationals and race it without fear of being protested, you're good.
With that out of the way, 95% of the work is done!
On the other hand, if you sail a one-design catamaran not
already listed in the SCHRS tables, you have a lot of work - and a lot of guesswork - to do in order to generate a fair and honest rating. Anybody who claims you can simply plug a few simple measurements into the SCHRS spreadsheet hasn't tried.
With regards to Portsmouth; the rating, once the modifiers were applied, resulted in the modified Tornado owing the N20c time! So, a boat designed in 1962 that is narrower, shorter and has pizza pans for dagger boards rates faster than a modern wave piercer with more volume, sail area and lifting boards???
With regards to SCHRS, I made an honest and good faith effort to measure my class-legal Isotope, and the official spreadsheet showed it owing the Goodall Viper F16 time! So, a boat designed in 1962 that is narrower, has a shorter mast, and shallow centerboards rates faster than a modern wave piercer with more volume, sail area, and a spinnaker???
I'd be happy to race under SCHRS handicapping if the system were anywhere near as simple to use or as accurate as its proponents claim - but it isn't.