As I was trying to say but the post timmed out before I could finish...

The formula is I=L/D² so; 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 1000-watt MH from 0? to 12? is about the same luminance say 140,000 lumens

Same light at 24? the luminance is now about 30000