Every sail manufacturer that sells sails online will have a tutorial on how to measure your sails. If you simply go to any one of them you will be enlightened and it won't seem so mysterious.
The percentage number you're enquiring about is indeed a comparison of the sail's area with the area of the
fore triangle.... foredeck, forestay and mast being the three sides of this right triangle. The confusion arises because a headsail is NOT a right triangle...therefore you must resort to your jr high school geometry to come up with an area for the sail... so... a triangle's area is defined as 1/2 the base times the height. Converting to sail lingo, the base of the triangle would be the LUFF.... and a perpendicular line from the LUFF through the highest point (the clew) would be your height... or LUFF PERPENDICULAR.
Okay...now here's the trick part... the length of the fore deck from stem to mast is called the "J" and is one side of the right triangle mentioned above. So.... here comes the jr high geom..... comparing the boat's "J" dimension with the sail's "LP" dimension will give you the same result as you would by calculating square feet of the sail and the fore triangle and dividing etc...
This means when you are looking for a particular size sail to fit your boat, you simply compare the J with the LP.... and voila!!!! ...... but wait.... think about this.... what happens if you move the LP up or down the Luff. The clew may end up very close to the deck..... or a considerable height above it.... Race boats often have very low clewed headsails, sometimes called "decksweepers" while cruising boats will often have higher clewed sails to allow for easier handling and better visibility....
That is the reason you want to understand how sails are measured and decide what shape will best fit your sailing preference.
To get some idea of how this works and where your boat will fit... try this site
www.thesailwarehouse.com/
