Depends on the corals you want to keep. If softies, farther away is good. If you get into LPS it should be closer, if SPS closer than that. Good thing is that it is adjustable. Pick a level and if it doesn't work out, move it accordingly.
I have a 30g with a 150w pendant. It's about 10" or so above the tank. I keep a couple SPS up on the top and the LPS and softies a bit lower. All seems to be going well with them. I will check tomorrow and see exactly how high it is.