I personally would calculate the lumens. If your HPS is rated at 95,000lm, then at 2 feet away it's 23,750lm, at 3 feet it's 5,937lm. So a 600 watt bulb has an effective radius of 2 feet. So any plants that are not within 3 feet will not receive the minimum 15,000lm assuming the plant is 12" high.