simple LED light rail question

[BoondockSaint]

im completely new to this hobby, and am looking to purchase 2 ai sol LED modules for my 70 gallon 36"Lx24"Hx18"D tank.

what im wondering is, should i be getting a 36"L light rack with the hanging kit, or 24"? theoretically, the 36 inch rack would be a waste, since i wouldnt be putting the lights at the extreme ends of them, otherwise light would be wasted in that it wouldnt even be going into the tank. therefore i should only need a 24" rack, centered, above the tank, right?

am i wrong in this way of thinking?