That's one measurement of a much larger picture. Useful spectrum lumens received is really what you are concerned with. From Maijuana Horiticulture: The Indoor/Outdoor Medical growers bible, by Jorge Cervantes recommends 10,000 output lumens for a flowering plant. I have heard lower amounts used though. Also, different 23 watt bulbs have different lumen counts. Many variables

I would try using two 23 watts per square foot. 50 watts per square foot, while a very basic measurement, still works within the normally used lighting types like cfls and hids. But the more watts you can put it, and still keep it cool, the better off your plants will be.