Hanging Metal Halide lights - what height?

[enigmatic]

Hi,

I've just bought a Sun Sun light unit from ASA and have hung it over my tank using the supplied kit.

I'm just wondering how close this unit should be to the top of the tank?

The heat from light has caused one of the lose pieces of glass that covers part of the tank to crack and break. In part, this is due to that glass being a lot thinner and not in the best condition (chipped, slightly cracked previously).

I'm thinking of raising the height of the lights but just wanted some input first.

Thanks,

Chris