The watts per gallon "rule" just gets you in the right ballpark for light. But, it seems that people are gravitating towards 2.5 watts per gallon, using AH Supply quality PC bulbs and reflectors, as a good amount of light. If you had just a plain white painted flat reflector, that would translate to closer to 3.5 watts per gallon needed. But, in any case it tells you that 1 watt per gallon isn't going to be enough and 5 watts per gallon is likely to be more than needed. I don't see a way to make it any more specific than that.