With HID lighting, the rule is about 40 watts per square foot (430 watts per square meter).
Go get another 400w hps IMO cheaper than a light mover the extra light has many uses even saving you cashAhh, good stuff.
Cheers for that, Merl.
EDIT: Got the main info I was after..
I wanted to check whether i'd be stretching the boundaries by trying to cover a 1.2sqm rectangle shaped scrog screen with a 400w HPS Digital Ballast on a Light mover. It would appear if i run the ballast in SL mode*, it'll be fairly close to optimum coverage.
*SL mode produces an extra 10% output at bulb
Go get another 400w hps IMO cheaper than a light mover the extra light has many uses even saving you cash