Alright bud,
I would say you need your screen as low as possible to maximise your grow space. Obviously you need to be able to get under your screen for maintenance/trimming, and you need room for the stems and airflow.
So you have a 400w HPS air cooled w/glass; thats great - you should be able to grow fairly close to the glass. I use 600HPS and w/out glass, so I'm not too sure on the distance you will be able to achieve from your glass to the canopy, but with good extraction and an additional fan at canopy level I would think you will be aiming somewhere between 4 - 6 inches away. Hopefully there will be no light bleaching at this distance.
Again, I am not exactly sure on the light penetration of a 400HPS behind glass, but I would think it should penetrate at least 12 -18 inches down into your canopy (strain / defoliation style dependant).
If you think I'm way off with the distance you can have your lights then adjust what I am saying accordingly...
What strain are you running? From seed or cut? These are important points to consider when setting up your scrog space and designing the scrog screen itself. I would make my screen from wood rather than PVC (more sturdy, plus is PVC safe to have under HPS at such close distance? I'm not sure). I would make my screen as big as would just fit inside my tent. I would use 50mm chicken wire for my screen, and a staple gun to attach it. alternatively use opposing eyelets to run string/whatever in a grid, again with 50mm spacing. I would also add wooden legs rather than attaching the screen to my buckets to allow easier access underneath.
This would be a good choice of netting:
http://www.ebay.co.uk/itm/like/221807592059?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=122&chn=ps&googleloc=1007064&poi=&campaignid=270621186&device=c&adgroupid=16885268106&rlsatarget=pla-146650298466&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=122
If you were growing a 50/50 hybrid, from seed - which you would expect to roughly double in size after the flip to flower, I would aim to fill 70% of your scrog screen holes before the flip. This way the initial stretch of the plant in the first 7 days of 12/12 fills up the remaining 30% of your scrog screen. You then allow the second week of stretch growth to go vertically (say another 12-14 inch above the screen by the time stretch ends).
Of course, if you choose a sativa it may stretch a lot more - so act accordingly when choosing when to flip to 12/12.
This should prove most efficient use of available space in your scrog set up, as if you lollipop under the screen, your light should then penetrate the full depth of your canopy above the screen. Thus your making full use of both horizontal and vertical planes of your scrog.
So to sum up, I would say allow 6 inch for gap between glass and canopy, 14 inch for your canopy, and your golden. This gives you 20 inch. Ideally you would set your screen at a 5 inch height, which is just about enough room to work with under the screen.
bOOm
