Use the actual wattage the bulbs consume, so the power saver bulbs labeled as 60 watt are really 13 watt bulbs. That helps, but don't be fooled by shooting for a specific watts per gallon number. The meaning of that number changes a huge amount depending on what kind of lights you use. T5HO bulbs, with the typical highly polished aluminum reflectors for each bulb, give far more light than T12 fluorescent shop lights, with almost no reflectors. And, power saver bulbs give much more light when mounted so they are perpendicular to the water, than when they are parallel to the water, and in either case it is less light than a straight tube with a reflector would give.
You could use a single 4 foot 54 watt T5HO light fixture to get about the amount of light you describe, but the intensity would be less near the front and back glass. Or you could raise that bulb several inches above the tank and get much more uniform intensity over the substrate, but less of it. That is what I would try.
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