This information is from the book titled, "Grow Great Marijuana" by Logan Edwards.
250w in a 2x2 area should be one foot away.
400w in a 3x2 area should be 1.5 feet away.
600w in a 3.5x3.5 area should be 2 feet away.
1000w in a 4x4 area should be 2.5 feet away.
But keeping in mind that...