I have 4 1000's covering 8' x 12'. They are spaced closer to the walls because the combined side coverage makes up for the non direct light in the center of the room. When i divided up the room evenly and hung them that way it seemed the edge plants suffered. Seems if you r only doin 12 plants you should be able to do 2 rows of 6 w/ the lights in a row as well. Whatever works for the room size and how your hoods cover the space.
Usually, a square arraignment of 4 lights over a square arraignment of plants will work best.
Think this:
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The reason being is that you only have 4 edges to worry about reflection with. The center of this square while not having a lamp hung over it gains overlapping light coverage.