well if its a new tank then you can try for as close as you can where its 24" deep id start with 8-10 inches and if it doesnt raise your temp drastically go for lower like 6-8.if you have a light now over your tank and this is an upgrade to a halide i would go a bit higher and work your way down to the above especially if you have corals.
ive never had this light keep in mind ive just had halides of different kinds over the years and this is what ive done so maybe someone who has this particular light will chime in a bit better
cheers