Growing distance from fluorescent lights

[terryros]

I use four tube fluorescent fixtures and predominantly semihydro growing methods for a mixed collection of intermedicate temperature Catts, Paphs, Phrags, and Phals. I have a light meter and can measure foot candles and know that light output from tubes varies as the tubes get older. However, I have a practical question for experienced artificial light people:

For the various genera do you have a simple rule for how far below the tubes you like to have the tops of the leaves for best growth?