The tank is 30 inches high, with the light 8 inches above the tank. Assuming 3 inches of substrate, the light will be 35 inches from the substrate.
The tank is 24 inches front to back depth, and one T5HO bulb cannot light up that large an area anywhere near uniformly. So, multiple bulbs will be needed, separated by about 8-10 inches so the light from one bulb doesn't mix much with light from other bulbs. If you used 3 bulbs, separated by 8 inches, each bulb would have about 8 inches of substrate to light, which is reasonable.
A single T5HO bulb Catalina Aquarium light will give about 25 micromols of PAR at 35 inches. A single T5HO ATI light fixture bulb will give about 20 micromols of PAR at 35 inches. So, unless you want low light, you will need 2 bulb light fixtures, 3 of them, 8 inches apart, 8 inches above the top of the tank, for 40-50 micromols of PAR, medium light.
If you don't like the idea of having to set up and adjust 3 different light fixtures, you could get Catalina Aquarium to make you a custom light with 3 groups of 2 bulbs, with the groups separated by about 8 inches, using this http://www.catalinaaquarium.com/stor...oducts_id=1820
as a starting point.