I have the same question so I'll tag along. I have a 72ga Bow dimensions 48Lx18Wx22H and run (2)250w 20k/XM moguls they are aprox. 12" from the water surface.
I have two 250 watt metal halides 12k that are 10 inches from the surface of a 75 gallon tank I don't see a temperature fluctuation at all partlybecause its the winter