The positioning of your lights depends on several factors.
1. Is this a fish only tank? If yes, then positioning the lights symmetrically above the tank for the appearance of even light coverage throughout the tank is sufficient.
2. Will you be doing a full reef tank where you will be keeping corals? If so, then the light requirements of the corals you're keeping come into play.
Two things to keep in mind are the "light spread" and the "light intensity".
Light spread is the amount of area the light covers at the bottom of the tank. You adjust your light coverage by increasing and decreasing the space between the lights and the water. Think of it this way...
Take a flashlight, turn it on and place it so that the light faces the table. As you pick the light up away from the table the area of light on the table increases and decreases as you get closer. Well this is the same with lights.
You want to raise and lower them until you are covering as much area of the tanks as possible.
Light Intensity is the amount of light measured at the bottom of the tank. Most people measure light in PAR using a PAR meter. I own a LUX meter so I measure in LUX. I shoot for between 8k-10k LUX.
So you would need to decide which corals you are keeping before more specific recommendations can be made.
