hmm... 35 year old (at least) design, now modded with spinaker...

Yup... you would be open...

Now...if you do 5 regattas a year... with 5 WL races each... that would get you a PERSONAL Supercat 15 with spin rating.... That means... half the time...you would sail better then your average... and half the time you would stink.... Of course... when you sail better then your average.. you would have sailed better then the SC15 rating against the fleet of boats in Florida .. This means you will win.... or be tied with someone for the win..

Now... at the bar... not so much love for you!.... Got a love Portsmouth...
Now if we PHRF your Portsmouth rating... you can politic about your rating at the bar for the next 5 years.. (or until either of the Daves takes the fork out of their eye and stabs you to death about the issue)

Enjoy!

Your alternative is to take the ISAF SCHRS required measurements of your actual boat and sails.. ... plug them into the published formula using the free spreadsheet.... get an unofficial rating. go racing... (if you want to race Portsmouth.... you can guestimate your portsmouth rating based on the SCHRS deltas)

If somebody complains.... tell them.... you are happy to present your boat to an ISAF certified measurer (Carla S in your area) when they pay for his/her time and convert your unofficial rating into an official SCHRS rating.

Transparancy and Authority.... essential to support for any handicap rating system.