Remember also that the inverse square law applies to point sources radiating equally in all directions. Fluorescent tubes are not an infinite line source (which would fall off linearly) but from the standpoint of a plant a few inches away they're not point sources either, even before you add a reflector. When you add several tubes side by side (approximating a planar source to a plant in the center), it's even more complex at those short distances. I still do theoretical calculations from time to time, but my recommendation if you really want to know what intensity you have in your setup is to buy or borrow a meter and measure it.
Re lumens, I find it a valuable number to have simply because it's one of the few that's almost always available. But as it's a measure of luminous flux (i.e., adjusted for the way human eyes perceive light, rather than how plants do, or of total radiant energy) it's only one of the things I consider.
Steve
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