Advice Needed: 400w HPS Scrog +Rep

Hi guys I could really use some advice to confirm some calculations Ive made. So Ive got a 3ft by 3ft tent (actually the tent itself is more like 4.5ft long but for the scrog this is irrelevant. Screen will be 3 by 3). Major issue is height. Only 3ft total height. So Im using 3 gallon 9 inch tall pots and plan on giving the ladies about 8 inches of screen before they get scrogged. Which is a total of 17in to the screen.

This leaves only 20 inches total distance to the light. Im picking up a 400w HPS with a dimmable ballast (so I can adjust it down if needed) and have a 200cfm fan pulling the air out through ducting. Ill have fans blowing under the canopy and another blowing directly on the HPS lamp (no cool tube available, but if necessary I suppose I could build one).

So my question is this: Am I going to be OK on the light distance from the screen to the light itself, or is this simply too short. I figure with the dimmable ballast I can dim it down to 250w if really necessary, but Im looking for the largest yield in limited space. Next time I will be building a much taller tent, but this time this is what Im working with.

Yes I considered running CFL the whole way but I really want that HID lamp on my ladies.

Thanks for any help. I really need it. Much rep will be given.

Temp control in some direct manner is gonna be necessary....easiest is to cooltube the light, exhaust the heat.....supply fresh cool air......
I currently run a 3x3 tent with a cooltube 400w hps and a 150w hps, open style....2 var. spd. vortex fans supplying exhaust .....with intake coming from my crawlspace....That temp is 30 degrees.....and although intake fan is on very low, it is still a definite need to keep temps in correct range....I may have a bit of overkill but I definitely have control over my temps.....and I grow some monsters........

So with no real world experience with a cool tube yet, is it really that huge a difference? I was thinking if I place a big fan close to the light pointing at my exhaust duct (which is up at the top near where the HPS will be) it would cool it enough. Making a cool tube seems like a great idea, but finding the parts in my current location (outside US) may be difficult.

ETA: You mean your temp is 30C right?

Not 30C....30F....I'm in a cold region/ US.....and yes, any way you can separate your light from the grow space and reduce heat is gonna be very beneficial.....keeping a stream of air blowing on your light will work according to ambient temps.....

Well this point is now moot. After going to the shop I found that it is impossible to get dimmable ballasts within my budget in my location. Ive decided on a 270w HPS which shouldn't be too hot for my cooling fans and Im going to supplement with 240w (85w x 4) of CFLs. Should work out ok. Grow diary will be coming soon. Total wattage of 510w pushing out 55,000 lumens.

Thanks for the advice Oldreefer!!! +repped