think average is 6-12" or so.
yoiu could always slowly acclimate corals higher to be safest.
Agreed. I have my Hydra about 8" off the top of my tank. Your corals will let you know if they are unhappy.
Have you been running this light, or are you just putting it on your tank?
I would imagine you are hanging the light? At least you will have flexibility, if you use an adjustable hanging kit so you can start higher and lower it if you aren't happy with how things are responding, growth, etc.