The whole watts per gallon thing to me is pretty much rubbish. What matters is the actual light output. Distance from the plants, depth of water needed to be penetrated, hours of light generated and the type of light. When dealing with flouros the length does not realy matter if the light tube goes the length of the tank. The light from a 1200mm tube over a 1200mm tank (38 watts) is pretty similar to the light from a 600mm tube (18 watts) over a 600mm tank. You get about 10 watts per foot of tube.