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 lumens so it??s 12? to 36? we have control of and the closer to the 12? mark the better because it provides the most lumens to the plant. To close and you burn the tips to far away and you give up luminance penetration (PAR). Next is Luminance loss. Most experts are in agreement that light intensity diminishes exponentially fast. About 16? is average if your plants get too tall just bend them over (if you bend them over the buds get more light and grow bigger). You could crop them to keep them short and bushy you crop between the 4th & 5th node early in the vegetation state for best results.