Still learning to tune my sails correctly. If a sail doesn't specify a range of luff lengths for adjustment, eg. 1 number, can I safely assume the one suggested in the spec's is maximum or +/- of that spec?
if there's only one number, then I'd suggest it's the margin either side of the manufacturer's recommendation.
Trousers I think what he means is that if luff is specified as 430 does that mean 425 - 430 or does it mean 430 - 435?
Sorry nosinka....they are all different so no easy answer.
At any rate, even if they specify a range of length it is still often wrong and the numbers on extensions can be different. So rig it right, not to the numbers.
That means talking to the experts with the same sails, or see if there is a rigging guide video online for that sail done by one of the pro guys or designer.
Failing that watch the rigging guides online by lots of other manufacturers and yours will be in the middle somewhere. Obviously for your wave sail don't watch race sails videos 
OK, it's as I guessed. The KA Kult seems to have over 50mm downhaul past the luff measurement, just wasn't sure how much more downhaul I could give it. I've got pics of a well set up Kult on a KA mast and didn't know how much downhaul was involved to get it to look like that. Especially on a non-KA mast. Also getting used to the idea that sails nowadays have more downhaul than 2 decades ago, I get nervous putting so much more pull on that downhaul.
Still have to rig & use the Ezzy, but I've sourced a tuning guide for that.
