yea thats enough space to use but that 400 will turn that small of a space into an oven unless you have good venting.
Also in that small of an area your gonna have to choose what to grow carefully so it stays on the small side.
Another thing is that you dont ever want to go 12/12 on seedlings or fresh cut clones. Seedlings need at least 3 weeks at 18/6 and clones need to have roots on em before you take it to 12/12 .
I been doing sea of green for years and when i use reg seeds i run 20 at a time. I can usually sex them by the 3rd to 4th week in 18/6. At that point i cut it up for clones or send it to the flower room. The clones i cut take on avg 2 weeks to be rooted enough to go to 12/12.
A seedling needs 3-4 weeks to reach sexual maturity and sex of the plant is determind by its genetics in the 3rd week.
If you put a seedling under 12/12 your gonna stress it out and get hermorphidites among other issues.
You dont want to cut clones from a plant in Flower 12/12 it will stress the clones by forcing them back to vegitative growth
Understand that nature plays a big role here and that weed has a "survive at all cost" safety built into it genes and thats called hemorphidites. When the natural light cycle the sun provides plants is changed from 18/6 summer to 12/12 Fall, plants say oh shit time to flower and seed. Now when you mess with natures cycle by Going 12/12 on a seedling or a cloan taken from a plant thats in bloom is going to freak the plant out and good chance to make it go hemorphidite.
There are quite a few threads around that show that seeds can be started on a 12/12 photoperiod without any problems, strains will respond differently, the goal is to get a sog type shape with little or no branching with the bud concentrated on 1 main stem cola
as for cuttings once rooted on a regular 16+ photo period the same principle applies, they can still be grown on a 12/12 photo period from start to finish, the result again tends to be a 1 cola plant with little branching
personally i would rather grow them for a week or two before i flower them
in such a small space with only 3ft height a sog could give the best yield imo with the right single cola type plant, you could fill the space with more bud
screen of green plants are larger although you could LST them to the screen to keep them shorter
normally screens would take up more space with leaves and branches and would take longer in the veg state to grow enough branches/shoots to fill the screen
you can get the plants very close to a 400w if you use 2 fans and a cool tube etc
it is better to have a fan blowing air through the lights rather than sucking air through them , the hot air is not good for the fan
when you blow air through the cool tubes the hot air of the lights does not come into contact with the fan at all, also you can suck the cold air from outside, or low down in the room, and blow this much colder air through the lights this has a much better cooling effect than using 1 fan to suck air through the filter then though the lights and through the fan itself
then using this same fan to remove all the air in the grow space too
the 2nd fan can suck out the stale air from the grow space and bring in new air from a passive inlet, you can put a carbon filter on the end of the extractor outlet to save space inside the grow area
peace