I have to agree that watts per gallon is completely false. Another factor is that the lack of penetration of light through water is logarithmic. That means that it doesn't take twice as much light to penetrate 600mm of water as it does to penetrate 300mm, it takes a lot more than twice as much. Also different types of light penetrate at different rates. Blue light penetrates better than red so in a deep tank the substrate can be lacking in red light. An example of this is why the marine enthusiasts use blue lights. The critters they wish to keep have evolved in blue light as the red will not penetrate. The joys of trying to find a simple equation to represent a complex situation. Don't forget than Grandad used to grow aquatic plants with an incandescent light bulb before floros were invented. Don't ask me how his Grandad got on with candles.