That's easy
http://www.hydroponics.net/learn/co2_calculator.asp

That's a caculator you will find helpful.

CO² during vegetation cycle will make them grow faster and larger BTW. If you run CO² then you also need to modify some other rules. Like temperature for example. When you use CO² (only when the HID lights are on) the temperature needs to be raised to around 85f for the plant to take full advantage of the extra CO²