http://www.hydrofarm.com/content/articles/co2.html here is a formula, depending on the size of your space and your resolve to use co2, you really need a meter/controller 3-400. Once you get the meter, then do the formulas, and you will see they are way off and go from too high to too low too often.
foaf is right on. You can get by and see some results by just using a timer and doing the math. But the controller/monitor set-up really improves things. There should be something somewhere on GK here with an exact formula for your CO2 injection. I think it was in the old FAQ, but I could be totally wrong. Just do a search.