One 1000watt hps over 3x6 tray

I have this setup in a bedroom. This is my first grow myself. My question is if 3x6 is too much space to be covered by the one 1000watt light? If so, should I keep the plants closer together and utilize less space on the tray? I have 10 rockwool cubes 6x6". I plan to put 1.5" rooted clones in and start the veg once I get them. My 1000watt hps is dimmable, so I figured maybe I should start the first week or so of veg off with 600watts and gradually increase to 1000? Also I have a 25gallon reservoir, but plan to only use ten gallons of that. I don't want to over water the plants... Does this sound like an ok setup? Please any advice would be greatly appreciated.

3x6 is fine for a 1000w I would do 6 plants 2 deep, 3 across. You could always hang some high watt cfls in the corners to even canopy lighting

As far as dimming your lights, I did the same thing with mine, started off at 50% and worked my way up to 100% within a week