The more light you can give a plant the faster and bigger they will grow. however, the limiting factor with metal halides or any HID light source, is that they put out a tremendous amount of heat,and if the plants closer than about 12-24", dependent on wattage you will burn and damage your crop.

Have you ever considered trying LED grow lights? High output LED lights offer the advantage of high light output without the heat and you can get increased crop size while not ever burning your crop, even if the plant touches the light source. Additionally, you can save upwards of 80% in energy while getting the same amount of light for your plants. Plus, you will not need the fan to remove the heat.

The above message, as I'm sure most viewers will know, is SPAM.

If you want to know how far to place any lights from your plants, get a Light Meter. As long as they are getting 30,000 lux, you are fine. If you are worried about them being too close, put your hand (palm side down) just over the top of the plants. If the back of your hand is uncomfortably warm - the light is too close.

Mike