100% main and Jib is supposed to be the standard.

A lot of manufactures fudge that and true displacement for better numbers on the SA/D ratio so beware the manufactures published numbers.

My Gemini might have weighed the advertised 9600 pounds dead empty, it doesn't now

My boat is listed at 340 on the main and 350 on the jib, which is a 150% roller, and then 490 on the screacher, which is a light air sail that PCI says is the equiv of 200% if it was a jib. Doing that math doesn't work out so good. because 280 is the I x J / 2 number and 280 x 2 is not 490

In fact, none of their numbers really works out.

The mast top is 40ft from the deck, and 14 ft from mast to furler. Basic formula Is I x J /2 , or 14x40 / 2 or 280 sq ft.

A 150% jib should then be 420, and a 200% should be 560.

It's all marketing hype

In fact Practical Sailor magazine (known for cutting through the hype) lists the Sail area of the Gemini as 540 sq ft - I have no idea how they came up with that as 340 + 280 = 620, unless the roached area of the main doesn't count either and it should because it's always in play.

Anyway here's a link for the basics:

Wildwood Sailing Club - Sail Calculator
it includes yawl and ketch designs.