NFT and Lights?

Hello Guys.
Quick question.
When the rockwool block is bursting with roots and i am going to put it into the tray and start the lights, how high above should they be?
and is this the same throughout the grow period?
Thankyou

Im a new poster on RIU, but an experienced gardener. General rule is as close as possible... but a decent guide would be-1000w-24", 600-750w-16 to 18", 400w-12". Thats just a general rule, but get them as close as possible. My temps of my canopies are between 74 and 77 degrees. I use 2-600w hps in plain air cooled hoods both on light movers in my flower chamber. (my lights are aprox. 8-10" from my canopy). You want to keep the lights at the same height from the top of canopy during the whole cycle. Or if you wanna go old school just put your hand just above your canopy under your light when warmed up. If its to hot to sit your hand under for 2 or 3 minutes its to hot for your ladies.

are the lights cooled? i have an exhaust fan cooling my 4 lights & i can get about 14-16 inches close with a 1000w. again, go with the hand under the light & measure then add 4-6 inches & you should be good for about a week, unless youre in a transition or the 1st week of flower then you may have to move them every other day.