I would do two radions. I believe the site reccomends 1 radion for every 24x24x24 section of tank. I have three over my 125 gallon (72x18x24) and they are so bright i dont need any other light in my basement. They are hung 7 inches above the water as per instructions.
The coolest thing about radions is their completly modular, so if one isnt enough you can always get a second one later on. Its not so much the depth as your how long your tank is. With just one light your going to get a "dark" patch around the light on the edges of your tank, assuming you put the radion right over the centerpoint.
You could build your reef right under the light, and put your SPS corals there, and have less light craving species of coral on the edges of the tank if like the look of just the one light.