Using CFL's and this plant is my 3rd tallest so actually the middle size. My closest plant is touching the lights (the plastic part of the light and isn't burning)
Straight answer: It's about 2" away.
It's getting worse, I've noticed it's spreading down the plant. It's really bugging me as it doesn't seem to fit into any nutrient problem descriptions.
So glad this isn't effecting the whole crop!