After coming from the 2015 Vapors, I had to put more time in the 2017 serie to get things tuned up. With other words : it was kinda challenging.
For all the sails I have used so far (5.8 / 6.4 / 7.1 / 7.9 / 8.6) the amount of LUFF length is about 1.5 - 2 cm more than the settings showed on the sails. Of course, settings are always just a direction, but if you give the 2017 just slightly too less downhaul, you will notice that.
First I rigged the sails how I normally do : only with the spacers provided in the sails and with the correct "optical" tuning. After that I go on the water making sure the whole sail gets wet included all the stiches. Then I controll the cam rotation on shore by clinking the cams. After that I try to get as much profil in the sail as possible with a normall cam rotation. All this went fine, only the amount of downhaul was a thing.
Biggest difference : put MORE downhaul in the sail at the moment you think there is enough. The loose leach reallt has to go down untill the third batten from below.
While putting downhaul in the sail, I look at the back of the sail just above the third batten from below. There has to come a kind of wrinkle in it, that goes from there, curved over the sail to the mast pocket. There the loose leach has to go almost parrallel with the mast to the top. See picture below.
After you rigged the sail and it is laying down, boom attached, the fourth batten from below has to touch the ground. If it does'nt and the sail performs good, you have found the lowwind setting. If you have the feeling you cannot close the gap during surfing, or the fin is too slippery, or the tail is feeling too light (not enough back foot pressure) you need to put a bit more downhaul to find the right minimal downhaul. The bigger the sail, the more sensitive the perfect tuning. I had to put most time in the 8.6m2 to get it good.
See picture below.
Untill now I used almost 1 downhaul setting for each sail, with a pretty amount of downhaul. When I make a gybe I hear the monofilm wrinkles in the top, that is what you want. If the sail is too full in the top, and it feels to stiff you have too check the upper cam. It must not give a lot of cam pressure on the mast. You must easily turn it with your hand. I even am testing the smaller sails without any cam in the top (my 5.4m2 2016 Proto is even used without a upper cam at all).