Okay. So you have an 18 square foot space you want to illuminate, first you need to understand the light requirements of plants and then the inverse square law.
Plants in seedling and clone stage require about 400-500 lumens per square foot. In vegetative growth they require about 2,500 lumens per square foot and in flowering, they require about 10,000 lumens per square foot.
The most important thing about light, is to understand that it diminishes rapidly the further away it is from the target. Now, As I'm sure you know, HID systems output a fair amount of heat and that means they cannot be used particularly closely to the plant tops for fear of burning them, so for a 400w (or 430w) system it would need to be about 2 feet from the plant tops. So if a 400w system outputs 53,000 lumens at source, it's represented as 53,000 lumens at a distance of 1 foot from the target. The Inverse square law is light intensity (output in lumens) divided by distance (distance the target is from the source) squared. So for a 400w system outputting 53,000 lumens we now know this will be outputting 53,000 lumens divided by 2 foot squared (53,000/2x2) which is 13,250 lumens at the plant tops.
So if we wanted to give the plants 2,500 lumens per sq foot for vegetative growth, we need to work out how far the lamp could be away from the source and that would provide us with the sq area. 53,000 lumens at 3 foot is (53,000/3x3) 5,888 lumens, at 4 foot is (53,000/4x4) 3,312 and 5 foot is (53,000/5x5) 2120, so we know that to deliver 2,500 lumens per square foot allows the bulb to be about 4.75 feet from the source which would illuminate a square area of 4.75 x 4.75 or 22.5 sq feet.
Light movers increase coverage by about 25%, so it looks to me like if you wern't using a light mover you would probably need 2 x 400 systems for flowering and vegging and perhaps 1 x 600 for both using the light mover. Now you have the math, you can work it out.
