I think I said this before but here it is again.

There are two factors to consider. Temperature is first; monitor the temperature at the average height of your plants. To close and you burn the tips to far away and you give up luminance penetration (PAR). Second 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 12000
Fan leafs block most of the light so nothing below sees enough of the light of day to do any good.

For me about 16? is average if my plants get too tall I just bend them over (if you bend them over the buds get more light and grow bigger)

I hope this helps

Peace