Rough calc of fluoro lumen output?

I was wondering if anyone had a decent formula for calc fluoro lumen output for various lanterns. Would you use rated watts, or actual power consumption? If actual, would you use the voltage under draw or nominal?

Ex. A 2AA fluoro draws 600ma on fresh alks.

1. At 3V x .6A = 1.8W
2. At 2.4V x .6A = 1.44W
3. The tube is rated at 3W by the manufacturer.

Which way is best? I've been thinking it was #2. But what about lumens/watt?

The reason I'm asking is because with all of these new led lanterns about, I was wondering where that left the fluoros in terms of output vs runtime.

take the ratings and multiply it by about 3 to 4 and that will be incan lumens so a 4 watt flouro light that says 4watt on the tube may only take 2 watts of power but will put out about 12watts or so of light compared to a bulb. A typical 4 watt lantern should give a lux 3 a run for its money and compete with a cree driven at 350ma better than people probably realize because no diffuser is needed.