This is a bad pic from my iPhone but you can see how mine are set up (8.5" to versa top, 19" to substrate, lots of floaters). Definitely low light at the substrate in my config. http://twitter.com/jarvitron/status/...730434/photo/1
I'll summarize what I said in a PM reply earlier
The spread on the floodlight is advertised as 120 degrees, I think it's more like 90 degrees on the short axis and 120 on the long axis.
Depending on how close to the top of your tank you want these (right on top of the lid for example), you want to pay attention to that. If you're using something that is very shallow for its length (like a 55 or a 12L) you probably want to set them up landscape-orientation and space them evenly on the top, one flood per foot of tank length.
If you're using a tank that is deeper than 12" (like a 40b or a 50 or 75), you might think about putting them in portrait-orientation, and closer together lengthwise, maybe a flood per 8" of tank length.
What I like most about my LEDs is how directional the light is, spill from the tank on to the floor or splash onto the wall is significantly less than any reflectorized incandescent or fluorescents that I have.
If used on a 12" high tank (the aforementioned 15, 20L) and right on the top, 12 or less inches from lamp to substrate, it could be very bright indeed with just 10w lamps. For 18+" high tanks (40B, 55, hexes and all manner of other bigger tanks) you might think about the higher wattage emitters.