I like how everyone makes their own rules. So far in this thread, we've had someone say that there is probably only one correct downhaul setting, even if the manufacturer specifies a range. Now we have a case where the manufacturer recommends only one setting, and folks are saying that wouldn't be true.
It will be interesting to find out why Gaastra only specifies one setting. It is in their best interest to maximize your enjoyment of their sails. No adjustments = no wind range = minimum enjoyment.