175 watt metal halide retrofit

[ezy33]

I just picked up a 48" 175 watt metal halide retrofit lighting system for my 55 gallon reef. Now I'm wondering what is the optimum hight the lamps (T-shape E39) should be above the tank? It's 48" long, 15" wide and 17" deep.

 

[ccCapt]

8"-10" above the water.