vertically hanging lights question on distance

If I was to hang a 600 watt or a 400 watt hps light with no hood in-between plants plants vertically. How close can the plants be to the light? Should I fallow the same rule of when I hang horizontally in a hood over plants 18"-24".

with a 400 you can place as close as 8" to 10" with no burning...with the six i would guess about 12- 14"

Much thanks reggaerican u have given me much advice I really appreciate it. I am asking mainly because I decided I would make a hydro spinner after seeing it in action. Mine will work with soil but am trying to incorporate other hydro method capabilities which is making it little harder but still do able. And I won't have the hight restriction of 26". My only concern manly was the spacing from light since once it is made plant pots won't be moveable. I was thinking 18" from the center of the pot to light so as plant grows and widens it will be about 10-12" away and if to close I can add a cool tube, my main concern is both heat and then being to far. Dose that sound like a good idea.

P.s plants won't be flowered no larger than 3 feet. I hope at least

and I am going to use the 600 watt hps...