How tight should my shade sail be?

We say tighten until you think it is tight then tighten some more! The sail should have no sag or droop and you should pretty much be able to hang off it or even sit on it (do not do this, this is just for description purposes!). Sail tension is key and paramount for safety, longevity and water runoff. Use your turnbuckles for final fine-tuning tensioning.