2,000 lumens per square foot, is about as low as you want to go. 2500 lumens per square foot, is considered adequate and 3000 lumens per square foot is considered optimal for the vegetative stage.
A good rule of thumb to calculate fluorescent lumens per watt is 85 lumens per watt. Some of the various types of fluorescent tubes put out more lumens than others. “Daylight” tubes put out about 6000 lumens, “cool white” put out 4000 lumens, “white” tubes put out about 3500 lumens and "warm" tubes put out 3000 lumens. There are also the colors of the spectrum that affect the plants. Blue is for the vegetative stage and red is for flowering.
Now this all sounds pretty good but it’s not much when you consider the power of Mother Nature. Sun light at the summer solstice, will generate in the area of 10,000 lumens per square foot.