T5 lights - distance from bulbs to plants?

[boytjie]

Hello, everyone! This is my first post to the board. Fairly new to the hobby here, but quickly (frighteningly quickly, actually) becoming smitten. My question might have been answered before, but a search failed to turn up a definitive answer. I have a bank of 4 T5 tubes over my growing bench - Paphs, Phals, a Brassavola, an Aerangis, and a BLC. What's the optimum distance from the tubes to the top of the plant? I know Phals and Paphs need slightly less light. Thanks for the assistance!