WPG is really a misleading way to determine the amount of light you need. PAR is a much more useful way of measuring light and varies with the type of light you use, T5's, T8's, LED's, MH and so forth. Gerryd is trying to determine what you want to accomplish and the time you want to dedicate to it in order to recommend the amount of light you will need.
How you plan to administer CO2, the types of plants, and maintenance all determine light and fertilizers. Think of it like this, light is the throttle (determines how fast your plants will grow), CO2 is the oil (keeps things running smooth), and fertilizers are the gas (provides the fuel for growth). These things need to be in balance to ensure healthy growth, especially light because it is the easiest to manipulate. Too much light and not enough of the other two and your tank will seize. Growth deficiencies deformities, etc.
You can get by on much less light than some places may lead you to believe.