a low-math alternative to Jim's method
Instead of marking the sail, mark the mast.As a starting point of reference, take the boat out on a day when you know the wind will build at a fairly constant level to a point beyond your normal comfort level. Begin with the main reefed so that the top of the leech exits the mast at a recognizable point, such as the top spreader. Now, keep one eye on the rudder angle and the other on the anemometer, which should be set to apparent wind. Go to weather. Every time the wind builds a degree or two, note the amount of weather helm/lee helm you're experiencing by noting how many degrees the rudder has to be over to keep the boat on track. It should be fairly easy to tell the points when the boat goes from being underpowered to being right in the groove to being overpowered. Trust the helm.So now you've got a number, such as "first spreader equals 25 knots apparent." And you can probably project what the ideal wind speed is for the top of the mast (e.g. "Full main equals 18 knots apparent.") Cool. Now measure the difference between the point where the leech emerges when full out and the initial reference point, such as the first spreader. Let's say it's 7 feet, just to make this easy. Now do the math. At 19 kts apparent you'll want to reef one foot below full; at 20 kts apparent you'll want to reef at two feet below full; et cetera.Now, before the naysayers (mathematicians) start barking about how the geometry of this system is imperfect, let me just state for the record that the leech is hollow, and that very few mainsails constitute perfect triangles, and that we're talking apparent wind rather than true, which is an additional fudge factor, and that the the final variable is the flatness of the main, which differs from one set to another. Let me also mention the caveat that this system works better with a fraction rig than a masthead rig, because with the latter you'll want to furl both sails equally to keep the rig balanced.Otherwise, it works. And it's fairly easy once you've figured the first reference point. Best part of this system, once you've worked everything out, is that you can glance at the apparent wind speed and then instruct the crew to bring out enough main so that the leech is X feet above the first spreader (or whatever your mark is) and you'll have nailed it.