Between 20-75 gals, 3WPG is pretty accurate. Below about 10 gals you need more light, above 100 gals you can get away with a little less. Factors such as tank depth, reflectors and water clarity should also be taken into consideration. These are general guidelines. As to How do you know when you have enough light, you're plants will tell ya.
FILSTAR Pimp #2
75 gal heavily planted,50/50 Black beauty,Eco-complete substrate, Pressurised CO2 with solenoid, ph controller, AB Reactor 1000, 330 watts 9325K GE PC lighting,Ehiem Liquidoser, 2-Filstar xp3 canisters.