Placing the fixture anywhere from 8-14inch above the water line is probably best, it's personal preference from what I can tell. Instead of starting high and slowly lowering the fixture however, maybe mounting it at whatever height you'd like, and (then slowly) start increasing the hours of lighting per day would be a better approach. For example, assuming your fixture has Actinic lights and Halogen lights on it, run the Actinic lights for 10-12 hours a day, and slowly increase their time On about 30min a day until you have them running at 16 hours per day. Turn the Halogens On for say 90min the first day or two, then each day increase the Halogen lighting by about 15-20min per day, until you have them only turning On for about 6 hours per day. Just an idea, this would allow your inhabitants time to adjust to the intensity of the new lighting, and then over time adjust to the amount of time this lighting is On. No matter how you chose to do it though, best of luck! 
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75g Soon to be Reef, Born 01-22-2008,
2 - 250w 14000k South Pacific Sunlight Metal Halides,
2 - 110w VHO Super Actinics,
ASM G-1 In-Sump Protein Skimmer,
200w Heater, 80lb Base Rock w/10lb Live Rock,
80lb Sand, 30lb Live Sand, 29g DIY Sump, Fuge.
Fish:
(1) Yellow-tail Damsel
(1) Six Line Wrasse
Clean Up Crew:
(16) Blue-legged Hermits
(5) Scarlet Crabs
(1) Emerald Crab
(20) Astrea Snails
(10) Trochus Snails
(10) Cerith Snails
(5) Nassarius Snails (sm)
(5) Nassarius Snails (lg)
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