light question?

am about to set an office tank, 75 g, 41w x 24.5H X 17.5 D, the tank comes with 2 T5 HO 39W EACH tube total 78w , so i bought another light fixture same set up make the total 156 w, am thinking to get a third one to make it 234w wich will give 3watt / gallon.
the tank is 24.5 inch H and the T5 are 36inch w 5 inch shorter than the tank.
should i get the third light fixture, or should i stay at 2w/g mark
co2 will be used
thanks

 

thanks Gerryd,
The reason am thinking to add another light bc of the tank H, which is 24.5 in.
At home am running 29g tank with 2x55 wt, i have both lights on at the same time for 2 h at noon , and the rest of the day with only one .
i was thinking doing the same except at home am using AH light system, not T5.
The original fixture will give 1 w/g which seems too low.
hmmm, dont know..
thanks