The wider you can space the tubes, within reason, the less the light
will decay with distance. The best you can do is to simulate a
uniform plane of light, which will then give no fall off with distance.
(As long as your distance from the array is small compared to the length
and width of the array)
An African Violet catalog that I have recommends a 6" spacing between
4-foot fluoro's just for this reason. You should space your bulbs as
wide as possible across the top of the tank. Of course, the more bulbs,
the better.
Rick