another 1000 watt hps question

I know that you are always supposed to run your lights as close as possible to the tops of your plants as long as your not getting them to hot or causing any other damage.

But what I want to know is I run a 1000 watt hps for flowering and I have heard the effective coverage area is 6'x6', but my question is from what height should or can I have my light to gain this type of coverage. I have started growing a few different strains and they are at different stages of flowering so I have a uneven top canopy as is.

For my setup I use 3 gallon pots (about 20" tall) to flower in, happy frog soil, and Fox farms nutes if that helps.

Keep in mind that even with powerful lights, the more plants you have under one bulb, the more they will block the light from each other. The canopy should be maintained by propping the pots of smaller plants up on top of things. Space the bulb 21" away from the tops. Coverage can be improved when using a vertical 1000w by creating a bowl shape with the canopy, with the tops of the outermost plants at about the same height as the bulb.