While the methods listed are not perfect for figuring out how much light you want over your tank I think captmicha is looking for a practical answer to "how much light should I use" rather than a philisophical answer or $300 light sensing machines
wpg, while it isn't accurate with every bulb type has been used for years with fluorescent bulb types and since the manufacturers make the fluorescent bulbs with the same light out put and watt requirements people have learned which plants grow with how much wpg.
The general consensus is you want 1-2 wpg of T-12 or T-8 light for low light, 2-3 for medium and 3+ for high light. If you have power compact bulbs, or T5 HO then you want 1-2 wpg for medium light and 2-3 for high light plants.
If you have an extremely small tank (under 5g) the minimum you want is about 10 watts for low light, 30 watts of fluorescent light for medium light and 50 wpg for high light. With a large 200+ g tank you can light it using less wpg simply because of the height of the tank/surface area. This is because the rule breaks down for extremely small or large tanks.
If you are determined to use lumens for some reason then look up the lumen rating of bulbs in the wpg range I mentioned. As stated lumens are balanced to the human eye which has little to do with what wavelengths are easily available to the plant. That said, total light output is more important than specific colors in general, so the brighter the light you add (even if its not ideal wavelengths for plants) will still be used because plants have many types of accessory proteins that capture the less useful light. Over time the plants simply make more accessory proteins and can use more of the "non-ideal light."