Watts Per Gallon applies roughly to T-12, and not too bad to T-8, though it is a little bit off. By the time you get to better bulbs and better reflectors it is so far off that it is not worth asking about.
Like asking how many miles you get from a bale of hay.
When horses were in use, that was a valid question.
Useless when 'horsepower' refers to a car.
If you have a 55 gallon tank, with one of the wood tall light hoods over it, with two 40 watt T12 bulbs mounted at the top of the hood, say 16 inches from the top of the tank, will you have enough light? What if you make some \_/ shaped polished aluminum reflectors to put under each bulb? Suppose instead you sit the bulbs on the glass top of the tank? Surely, no one believes that 80 watts of T12 light defines how much light you have for all of those variations.