For clarity I don't run the main and 160 in 30+...just an example to try to bracket in the sane limit for my boat. I have four headsails. A very light 160(down wind "gennaker", maybe), a 153% Genoa, a 130% Genoa, and a 110% #1 jib(wraps the shrouds by 1' when pulled tight).t what wind speed, approximately, would I use each head sail? When would I douse the headsail and go to main only? When should I reef?
Ideally, you use the right combination of head and mainsail to balance the boat. You have to find out what that is for your boat in your conditions. If you have a reef in the main and your smallest headsail flying and are still getting beat, it's probably time to head in, after dropping one or the other, or both and motoring in. On my boat, I consider my jib my "main sail" and tend to reef the main first.
You seem to want formulas, which sometimes don't lend them selves to sporting activities (lots of variables). It reminds me of once leading a novice Sea Kayaker on a crossing. He couldn't make it across, and when I came back and asked why, he informed me that his GPS told him he was moving backwards at 1/2 a knot. I told him to put his ******* GPS away and dig in with his paddle! He did, and we got across!
The book I recommended earlier is an excellent start. It explains why sails and sailboats, perform the way they do, in simple, graphic detail which we don't have access to here. Once you have that knowledge, formulas don't matter. You can see how the boat is responding to the wind and tide, and can adjust accordingly. Start underpowered, and work up. When in doubt... reef. If you are consistently sinking the rail, or need excessive rudder (more than 20 degrees), you're probably over powered. Learn your boat, simple as that. Oh...and you have a girlfriend who is not afraid of heeling? Consider yourself very lucky! Many never truly get comfortable with it. Whats the formula for finding a lady like that?