HIya!,
I run about 5 watts per gallon on my 55gal. I have zoos mainly in the upper half of the tank, but thats just because its where I wanted them. My lowest colony is actually under a ledge. They seem to be doing well there, but the reason I placed them there is to get them to grow out onto a certain piece of rock that is on the edge of light and shadow. I think where they are at they get plenty of reflecting light from the glass and off the substrate.
In your tank, if you want them low, then place them there. If they don't open up after week or so, relocate them higher into the water colum. If there stalks become elongated, then they are reaching for the light. They may not be getting enough. Again, relocate them high in the water colum.
One thing to keep in mind with elongating stalks is competition. They elongate their stalks if they are in a tightly packed group or competing for space with other corals...like star polyps or whatever is near by.