The common rule here is 10cm ( 4inch ) distance for every 100 watt hps. Works from 250w to gavita 1k-watters. 60cm for 600watt, 1meter (40inch) for 1kW. 4inches less is doable, lower than that is above all unnecessary.
Search the username on some other large grow forums for a lot more info/facts.
He posted good info without the 'h' in the username at icmag. Probably also at opengrow.
Will be making one myself sooner or later, I got a gavita hood (tripplestar, pretty neat allowing for different beam width settings basically but not good enough for 4x4, more for 3x4) and use a phillips magnetic ballast. I think I will got for the new 600/750 series, need to keep it small to keep a low profile (rental, electricity use...)
Is sannie and crew doing ok man? Any recommendations as far as strains? I'm going to crack a pack of caramel candy kush the pics they have aren't very good I'll see if I can get some better ones up.
The fact they are not supposed to be directly air-cooled doesn't mean they need to be indirectly cooled. And yes, running at a certain temperature range is makes a difference in output and even skews some tests.
I don't really know, the crew has changed a little over the years. That admin now working for gavita, new admins, new mods, I haven't been active there for over a year. Sannie and that admin used to post good stuff but aren't that active anymore, and I got the impression they care more now about opengrow than their dutch forum.
)
Images are watermarked but dutch forum, no competition for riu so didn't bother removing it...
not a great run, better example, same grower, different tent:
yeah obviously not fed probably, had rot too, but damn.. massive yield.
Yes, fog. Designed to run in very humid greenhouses...
day 44:
Bulb eventually at only 24inch from plants, plants 100cm (3 feet tall, 80% of that is hard buds). 
yellow harvest... they call it "ripe".
-> grow, harvest, sell, make money, buy new:
<-controller, allows them to dim automatically when it gets to hot (like over 86f)
I got one in a metal box in my closet for a year already, 1200m3h, not connected yet cause it's overkill..., "softbox" prevents snitching neighbors from hearing it. Gets noisy, hums, when dimmed with cheap dimmers.
They probably are number 1, overall, but that doesn't mean everything in their line is the best out there, at least, I think so. Like I said in one of the posts they are amongst others popular for doing a better job at running the DE than the creators of the DE (phillips) (because of their fixtures, ballast+hood), BUT that doesn't mean they are the only one. If I remember correctly a sun lec hood provided a better par output on a 36x36" footprint than a gavita at growershouse.com (see test and review section there, no idea how reliable, but seems proper).
Like I said with my pics of those inline fans, there are exceptions. That 710cfm version of the hyper fan from Phresh (I use their filters) is 1200m3h and could be a very nice fit for a 1kW hps.
is there an A/C used in lets say using 6 of those or is it total ventilation system? and at 24 inches can 1000 watts be fully used? and some one was telling me at running even less then a 1000 watts he was seeing better lighting then his previous 1000 watt set ups. would you say that is true. sometimes its hard to believe gardeners around my area. every one seems to shoot their mouth off about this and that.
For 4 or more it''s not uncommon to have an A/C unit. The "Opticlimate" is a popular one for larger setups, does it all.
thats fucking boss dude. thanks for that. have you seen one in action? and that can handle lets say a 4 burner or 8 burner? wow man. thats fucking cool. and here i bought a 3 ton split system. going to google the price now on those.