If I am reading correctly you are saying that the spinnaker luff is longer than the distance from where the head connects to the mast and the end of your pole are. I do not think mine is that way as I like to keep a really tight luff. I think 2.5% longer would make too loose a luff while sailing. I do slack it a bit for light or heavy air. If I read this wrong then forgive my post.

The girth of my spinnaker is not at the minimum 75%. As I recall it is closer to the 80%. I remember, when measuring it, that there was a fair amount given up to that number. The foot of the sails are getting smaller with the trend towards higher aspect chutes. Maybe the higher girth number is to assure that the spinnaker is still large enough, 21 sq meters for the Formula 18s.

Later,
Dan