thanks for replying guys my tank is 12 deep and my light fixture would be 4 inch above tank.
You will need to specify if you are using T5NO or T5HO as well.
As mentioned, the lighting fixture that you are using will also affect the amount of lighting you have (i.e. whether they have reflectors, etc)
Here is a more up-to-date guide written by Hoppy for your perusal.
Lighting an Aquarium with PAR instead of Watts