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 48? 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.

For example:
A 400-watt MH from 0? to 12? is about the same luminance say 55000
Same light at 24? the luminance is now about 30000

Still good

Same light at 36? the luminance is now about 1200
Fan leafs block most of the light so nothing below sees enough of the light of day to do any good.

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.