My rough guide. Reef 1, 2 & 3 ..10% , 25% & 40% others go more.

Lexmark said..
A Quick question
Are the reefs in the main at percentages of the sail area ??

Yes...break out the calculator and apply the above percentages to get a lineal measurement , is that what you are asking ?

As each boat handles different, there are no hard and fast rules, so you need to plan ahead. Once you are starting to get the main inside out and the boat is shipping water over the sides, you are way past the point where you should have put in the next reef.

On my East Coast, a mast head sloop with fin keel, our full racing genoa will drive the main comfortable until about 15 knots. Our #2 on the furler will go to 25, but the main wants a reef about 20. We haven't found the limit of our #3 (not brave enough), but I had 2 reefs in the main at 35 on Pittwater. Our old main had 3 reef points, our new one only has 2, so we have some more learning to do.

If you are thinking about a reef, it is time to put one in. Reef early and relax. When it passes, pull out the reef and relax some more.

Probably best to ask your sailmaker. I found it best to take my sails to them, but I found some of the more expensive lofts will come out to your boat to do the measurement.

Reefs are classified as a percentage of the luff length not sail area.

For racing Cat 5, min 25% reef.

For Cat 3 and 4, min 40% reef.

An important factor with regard to the position of reefing points is the strength of the mast and support. A deeply reefed main on a rig that is not designed to carry a load mid mast may in fact break. Baby stays and inner stay are often used to oppose forces of a reefed main just like check stays or runners oppose forces of a staysail