So how close you can place the light is really determined by the temperature at the top of your plants. 12? or less is all the same. So it??s 12? to 48? we have to balance between heat and lumens penetration and the closer to the 12? mark the better without burning the leaves. If there is too much heat coming from your light then add a glass heat shield and heat exhaust fans to either suck the heat our or inline to push the heat out. How long you vent tubs are will determine weather you push or pull the heat out. You are better off if you blow air through your hood. This will allow you to get the light as close as possible but watch the temperature. Use an indoor/outdoor temperature probe to monitor the temperature at the top of the canopy. You can raise the height of the other plants with a box to try to make an even canopy. Watch out for the seedlings they are better off with clones under fluorescent light. That way the international temperatures of the leaves do not dry out and kill the plant before it gets a chance to survive.