The WPG 'rule' is really a loose guideline to judge approximately how much light your going to need over a tank to reach a certain level. If you were to go 4 WPG on a 100 gallon tank, even with the best Co2 and ferts, your most likely going to burn out the plants. This is because the WPG rule breaks down for larger tanks, and for very small tanks.
Tab hit it on the head, it is very subjective. And the levels of light all depend on the types of plants you want to grow. I have 1.06 WPG over my 75 gallon, but all my crypts, ferns and anubias grow wonderful.