If you are using pendants, cut a hole in the top of your canopy and mount the bottom of the pendant flush with the top of the canopy. This will keep ALOT of the heat out of the tank. Using this method, my MH actually heat my tank LESS (2-3F) than my retro 4x96W PC did.
Mine are about 6" off the water (pendant), but the bult itself is about 8" off.