ok in the simplest terms possible, and not seeing your system,,,, vent from the top of your room, and keep the ppms under 1600 to be on the safe side. (ive read that in late flower you can bump them up too 2000, but IMHO thats pushing it) if you pull air from the top of the room you wont pull very much co2 out. co2 is almost twice as heavy as oxigen. and if you run higher temps you can use more co2. and im sure you know you really need to have a meter handy to get it exact, but if you dont just watch what your plants tell you. start small and move up alil at a time untill they look unhappy then back it back down to a happy zone.
if money is not an issue you can get a computer controlled co2 injector that will keep you room at any given ppm.