Metal Halide Height

[sleeman]

Hey All,
I'm just about ready to put up two 250W MH's over a 72 gal tank. Any suggestions as to how high above the top of the tank. Is there an ideal height? It is built into a wall, so I have about 2 feet above the top of the tank to play with.
Can I leave the glass tops on to cut down on evaporation, or do I need to remove them?
Any suggestions would be aprreciated.
Cheers,

Al