what is the optimal distance between my plants and my T5 lights

[tragic johnson]

my veg room is now stocked full of T5 light fixtures, so my question is how much distance should i set between my canopy and my lights i have the same question concerning the light over my clone rack

 

[juniorgrower]

I keep mine an inch or two above the canopy. With clones I keep them about 18 inches above the tops.