How close should the light be? I have it at 12" right now, should I lower it?

I added a screen this weekend to start training it. For my cabinet that is 18"x48" I have almost a 50% screen coverage with the one plant and I have a dwarf plant that for some reason is 3 weeks behind. Is there a majic number that I should wait to in veg, or do you think I should switch to 12/12 now? My thought is that if this thing triples size, I am going to fill the cabinet pretty good with my 1 1/4 plants..

I am thinking for my next grow to remove the cabinet and just use the room I have the cabinet in and add a 600W to my 400W for a 1000W total. I would like to run 4-6 plants outside a cabinet to get large plants.

Thanks for the feedback.