Short answer: You don't. Watts Per Gallon is not a thing. At least not with modern lighting and new tank dimensions.
Have you checked out the lighting sticky to read about PAR? There are a few links there that will help you determine your light levels when using CFLs.
You can still figure out about what your par is by knowing about par. You can't just read the stickies though, you have to comprhend and understand what you are reading. When you understand it, it is pretty simple and makes a lot more sense.
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