Dave, the way that light intensity varies with the lamp height is really quite complicated. The standard "internet logic" is that light intensity drops off with the square of the height, in other words doubling the height would make a quarter of the light intensity. This simply isn't true. Light from a point source drops off with the square of the distance. A fluorescent tube isn't a point source, it is a long line. If it was an infinitely long line then the light would drop off linearly with distance, twice the distance, half the light. More importantly, a fluorescent with any kind of reflector is not a point source. The light intensity from a good reflector drops off very little with distance, because all the light remains focused on a fairly small area. Think of a perfectly focussed light source like a searchlight or a laser. The light intensity at one inch is almost exactly the same as the light intensity at ten feet.
The linked thread contains actual measurements and you can see that the light drops off much more slowly than the inverse square law. Notice how it drops off more rapidly with distance. Going from 3" to 6" (double the distance) produces a 33% drop, whereas going from 6" to 12" (also double the distance) produces a 50% drop. This is because a fluorescent tube (without a decent reflector), or a pair of tubes in this case, looks more like a point source when you are far away from it.
In practice, only you can know how the light intensity varies with height in your setup. Ideally, you would measure it since our eyes are hopeless for checking light intensity. The one thing you can say for sure is that the intensity drops off a lot more slowly than much of the lighting advice would have you believe.
Here is a link that might be useful: Foot-candles from a T8 fixture
