Pretty much goes by lumens/sq ft (or better yet PAR). Depending of the efficiency of your 68 watt cfl
(which I'm guessing typically is 60-85 lumens/watt) you are getting about 4,080-5,440 lumens.
That's just about enough for 1 sq ft coverage per light. 3,000 lumens/sq ft is minimum, 5,000/sq ft...