The number of watts you use isn't significant. The number of T5HO bulbs, and their spacing, plus the quality of their reflectors and ballasts is very significant. 60 inch T5HO bulbs are 80 watt bulbs. If the tank is 72L x 18D x 22H, you only need a single T5HO bulb providing light to any area in the tank. With an 18 inch front to back depth tank, 2 T5HO bulbs, separated about 6-8 inches should light up the tank pretty uniformly, and would give you about 50-60 micromols of PAR with the light sitting on top of the tank. Raise it 2-3 inches and the intensity drops to about 40-50 micromols, which is a very good light level to use. You can get this light from Catalina Aquarium, as a custom light, made using a 10 inch wide housing designed for use with more bulbs, but they will build it with just 2 bulbs if you ask for it.
I think it will light the ends of the tank adequately, too.