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Seems 12-18" is average.
I'm running a hybrid fixture of 4 T5s and 3 Hydra 26HDs over a 4' wide 20" deep.
With LEDs alone I'll run it 12", with the T5s I kicked it up to 16".
The question is it bit to general. Type of light. How many you have and how many watts along with what you are keeping are all big factors. But I will say this. Modern LED can carry a whole lot of lumens. And most people run them close to fifty percent. Plus the higher they are the better the different color Chanel’s blend. With all that said The higher the better. Perhaps that’s why the sun is mounted 94 million miles away lol.