Yes
Yes that's a good ballpark. Your I dim is what you go by for the luff/leech length. I've heard that you can exceed the I dimension by something like 4'. I'm not saying that's a good idea, or race legal. I dont think the halyard will give you any extra length. It has to do more with the curve of the luff under sail. also, the hypotenuse of a right triangle is longer than the legs (I and J). I know the foot is 1.8*J for PHRF racing where I'm from. It could be similar everywhere.If you're going with a new cloth... I'd find out what the racing rules state for max dimensions. That way if you ever race, you wont be penalized.