Well the easy way is to use a controller. You set your PPM and then set the exhaust cycle the controller does the rest.

For me I like 1800-PPM total, room air is about 300-ppm so I add 1500-PPM to reach the 1800 number. You can use timers you just need a few of them.

I run the CO² generator 10 min to reach the 1800-ppm every hour and 1/2. I remove the air about 10 min before I turn the CO² back on and start all over again. For me I vent differently then most do. I use 2 motorized dampers so my carbon scrubber can put the scrubbed air back into the room or exhaust it out of the room. Depending on the cycle it's set for.

Example:
8:00 am lights on CO² running.
9:30 am switch the dampers from returning the scrubbed air back into the room to vent to the outside.
9:40 switch the dampers back so the air returns back to the room.
9:41 turn CO² generator on
9:51 turn CO² generator off
11:21 exhaust again and start all over.

The AC deals with the heat so I don't have problems with heat build up. Fans move the CO² around and the idea is to fill the whole room not just one area. The plants take what they want. You can rise the level above 2500-PPM for a day or so if you have bug problems too. CO² makes a good insect killer, the plants will use up what they want so don't worry about over dosing them on CO². You do need to play with your timer settings and know your plants and room.


http://www.hydroponics.net/learn/co2_calculator.asp

This calculator may be helpful to you.