I guess I will piss you all off then and rain on your parade.

My light meter and experience tells me A 400w HID light is only really good for about a 2x2 square or you loose most of your light penetration.

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.

Do the lumens math it does work.

I=L/D²

Lumens received are a measured in watts-per-square foot or foot-candles. Calculating foot-candles or lux is a more accurate way to estimate the amount of light plants receive. This still only measures how much light you plants have available but not how much your plants use.
Light diminishes exponentially fast!

The closer your plants are to the light the more lumens they receive the better they will grow, as long as the heat from the light does not burn the foliage


1000w HID lights are good for about 3x3 but you can push them to 4x4 but I would not with out overlapping lights.

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

Still workable and not bad.

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.

That is the quick answer, do the math before you place your lights. If you are using a hood then check the light pattern in a dark room before you put your plants in there so you know who will get light or not.