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I'm a newbie but think I can help you out. They say 3 to 5 watts per gallon, but being you only have LR I would go with the lower end. You will eventually have coralline algae on the rock and maybe some other things. What kind of bulbs came with your tank ???
LIve rock does not require any specific type of lighting except to keep and or promote coraline algae growth. If you wish to promote the purple coraline algae growth then I would go for something like 2.5 - 3 WPG if your looking at florecent lighting systems. A 50/50 bulb or a mix of 10K bulbs and actinic bulbs will also help.
Corline does not go with just lighting but also needs good levels of alkinity and calcium in the water.