I'm just kinda throwing this out there...in the hope that it might be of some use to someone, somewhere...
What we're looking at is a 4'x8'x32" high seedling table that I built in the greenhouse. I've velcro-ed bubble foil wrap around the table so that I can open and close and get in there with relative ease. The plan is to use the space under the table to start seeds. Inside the space are 2 sets of 36Watt T-8's (that give off very little heat) and a 250Watt MH bulb suspended in the center which gives off significant heat. The MH bulb is on a timer and only comes on from 12:15am to 7:30 am (when electricity here is 4 times cheaper than during the day). Night time temps here have been at or below freezing for many weeks/months now, so the ambient air temps in the greenhouse run about the same(1 or 2 degrees above whatever it is outside at night).
Except that under the table night time high temps are generally between 24 and 30 degrees Celsius. Night time lows even without the MH running are still 10 to 16 degrees Celsius which is way better than I thought they'd be. Of course ventilation interferes with the temps, but you need ventilation. I left one of the pepper plants in there for 4 days with no ventilation just to see what would happen (I knew it couldn't be good...it wasn't) and lost a couple of the smaller fruits. But I only ventilate during the day time when temps are higher, with no ill effects thus far I understand that this may not be the case with tender, damping off prone seedlings. There are enough little gaps between the velcro strips so that some air escapes even when the set up is 'closed'. I've been using this for about a month now, and have been/am comparing the temperatures under her a few times a day to the outside and ambient greenhouse air temps. Usually around 10:am then 3:00pm and 2:30am respectively, and am pretty satisfied.
The pepper plants are some leftovers from a vermiponics and organic hydro "experiments" and are doing well for a sub-tropical type. There were more of them (doing well too), but they've been sacrificed to various other "experiments".These are/have been fertilized using a humonia, wood ash, water mixture.
I know none of this is very scientific, and this whole deal was based just on 'guess-timation' and some previous experience with HID lights and bubble wrap, but it's working out wonderfully thus far. I may try a much larger 'greenhouse within a greenhouse' next winter when hopefully AP will be in full swing, and use the four sets of 400Watt MH's I have in that "seedling and post seedling area. I doubt that the info I extrapolate here is 100% scale-able, but my current thinking is that it might get you close enough for hand grenades . (Volume of space/air by Wattage of lights = a given air temp, it just wouldn't seem to work that way with all the different variables). Anyways...
I would not suggest using any HID's above 250Watts (400, 600, 1000Watts would all seem out of the question) at this distance (at or under 2 feet) from your plants, particularly young seedling as you will in all likelihood damage (i.e kill) them. (Pictured here is one of the T-8's, there is nothing directly underneath the MH yet).