I am no expert but I can pitch in a little. Hoppy is the man when it comes to this.The wpg rule is out dated now. At one point that is all we knew for a go by but technology has changed and there are so many different types and variables of lighting now. The wpg was based on everyone using the same lighting fixture for pretty much the same tank which back then were fluorescent tubes, then came compacts, metal halide, led, and cfl and so forth. A fixture can have the same wpg as another on a particular tank but the light itself would react differently given it has different properties. Thus the best way to measure light now is the PAR value.