Watts per gallon is the worst measurement for light just because of the
reasons you list. It does not take into account the depth of the tank
or where in the tank you are putting the creatures.

What is the best measurement for lighting then?

I supposed the coral under the bulb is the winner regardless of the
watts/gallon rule or the size of the tank, but how much light would be
acceptable? If I place the coral under the 30watts bulb, he'll be getting
not 6w but 30w, right?

What you think?