3x3 is about as far as you can effectively push a 400Watt hps.
You want it as close as you can get it, without burning your plants. With good air cerculation, I keep mine at 10" from the plants directly under neath it. The plants at the edges don't do so well, as they are farther away from the bulb.
Hang a small thormoitor at canapie hight. A good trick is to hang it from the light reflector, that way you can easly see when it is time to raise the light, beacuse the plants would have grown past it.
I was growing DWC, so my set was a little differnt. If you have a 3x3 tray obviously you can't make it wider or longer, so If you want to sqeeze more plants in, make it deeper.
3x3x1 with a short Veg, 12 plants. Prune the bottom third. (Note: I have not tryed this yet, but from what I have learnt, this is what I will be doing next grow.)
Hope that helps you.