150 watt HPS - distance from plants?

Assuming my ventilation is very good, what is the distance at which I should place my 150 watt HPS from the tops of my plants? +rep for smart response.

i keep my 250w about 5-6" away with a fan blowing across the plant tops
should be able to do the same with a 150w, just check the temps with your hand to be sure.

I use a 150W HPS and I have 3 gals underneath. I find that if I keep it about 12" from the tops I can get enough coverage for all three plants while still being close enough. I have had it as close as 6" but I wasn't getting as much light coverage. Just find that happy median. You can hold your hand above the plant and then move your hand upward towards the light and feel that point where it starts to get real hot.