Rick, that formula only applies to powerboats
Powerboats don't heel (when they do, it's called listing--not good)...so as long as you can seat 'em all and keep all their body parts inside the gunwhales (and that includes inside any cabin) , it's safe to carry 'em...all you have to do is distribute 'em evenly to keep the boat from listing. So on a sailboat, as long as the sails are down and the boat's only ferrying people to an island picnic under power, your formula works. But on a sailboat, under sail, load distribution and the ability to shift that load as heeling requires makes it a whole different ballgame. The bigger the sailboat and the deeper the keel, the less a few hundred extra pounds on the lee side matters...but 700-800 extra pounds (6 more people than the boat can safely carry) who can't easily move the high side when a small sailboat tacks could capsize it. And the 12 people your formula says a 23' sailboat can carry couldn't get out of each other's way if their lives depended on it...which it does.