First time hydroponics guy here. I have a AeroFlo 36. I started from seeds (Sucrine Lettuce) 21 days ago in Rapid Rooter, just planted my 3" plants with clay pellets in 3" net cups. I have adjusted my downspouts so the water level is 1/2" above the bottom the the net cup. The root strands are hanging slightly beneath the bottom of the cup. I have 600 watt Metal Halide bulb about 4 feet above the plants.

I'm using General Hydroponics nutrient Flora Series, right now since the plants are so small I've started with 550 ppm (1 part micro 1 part gro 1/2 part bloom) as I get growth will bump up to over 1000 ppm.

18 hours light / 6 hours rest... pump running 24/7

So here's my questions; How far off the plants should I have the light? at 4' the leafs are not warm to the touch nor do they appear dried out. I'm curious if I can get better growth with the light closer?

Anybody see any problems? Please feel free to comment on my set-up.