There are several things to consider when mounting the lights over your tank.
1. What is the aqua scape going to look like?
Will there be 3 separate islands? Or continuous rockwork from one end to the other? If continuous you may want to consider mounting them higher to allow for more blending from one fixture to the other.
How high will the rock work go? If its higher may want to consider mounting them higher so there is no spotlighting.
2. Vega's have 80deg lens. So the light will travel in a cone at a 40 deg angle from each side of the LED's.
To help with placement of the fixtures Whats the footprint of your tank? To avoid too much light spilling out of front of tank, it will require a little mathematics. And a right angle triangle calculator
Mounted 6" above the tank, each led will create a 10" dia circle on the water surface
8" - 13.4" circle, 10" - 16.8" circle, 12" - 20.2" circle
For a X length x 24" wide aquarium. The light will travel a depth of 14" before it reaches the front/back glass (12"). Assuming that the light was mounted at water surface. 6" above, then the light will start to spill out the front glass 8" below the water surface.
I have a 8x2x2. I run 6 Vega Colors. I have 3 main islands so I run 2 vegas over each one. The lights are 3.5-4" off surface of the water. So 10" water depth before it spills out the front pannel.