Ark,
The wpg rule is like the dropchecker. It's completely bogus but it's the only thing we have that someone can look at and instantly have an idea whether the lighting is on target or not. At the end of the the day it's either use wpg or study Einstein's Special Relativity for the next 4 years at Cambridge University. Now, which would you prefer? For tank sizes between 20 gallons US to about 100 Gallons US it's a pretty good rule of thumb. These days it's more apt to be relevant to T5's and MH. There is more breathing room with T8.
So that if you have a 30 gallon tank and you said you had 2 wpg T5 or greater I can tell you without a shadow of a doubt that you should be injecting CO2, especially if those lamps have reflectors. Again, crude but effective. This does not mean that you can't run an un-injected 2wpg 30 gallon tank successfully, only that injection would give you better odds of success.
Once you get beyond 100G or lower than 20G then it's a matter of feeling your way. You can probably apply the wpg rule at the low end but the available bulb choices are not as plentiful so you wind up using 11 watt or 18 watt bulbs and so forth. At the high end you need to refrain from the wpg rule if for no other reason than that your electricity bill will send you to the poor house.
This is not to say that you can't apply the rule for large tanks, but if you do there are brutal ramification in big tanks. The filter throughput rule becomes more important, the CO2 injection techniques become more difficult and the dosing rules also start to break down. So lets say you have a 300 USGallon tank. Normally 400-500 watts T5 or MH lighting would be the most you'd want to use - but that's less than 2 wpg and yet it still counts as a high light tank with CO2 injection compulsory. You'd want that 3000 gallon per hour throughput even if most of it were just recirculation because you've got to move that water around. The CO2 consumption becomes tedious and the injection methods become difficult.
In fact wpg breaks down terribly if filter throughput is poor for any tank because you must deliver nutrients and CO2 to the surface of the leaf with greater efficiency as the amount of light increases. So wpg is sort of an all inclusive rule of thumb that takes into account the volume being illuminated, the nutrient delivery and the CO2 injection. It is not just an arbitrary rule based on wattage alone.
Not sure if this helps.
Cheers,