I'm having the same challenge with my next cabinet design. So far I've come to the conclusion that, without spending major dollars on high-tech controllers, my best bet is going to be using a recycling timer to activate my exhaust for 1 minute every 10 minutes. This way I can be sure that I'm running 90% efficiency at least. Also if your tent is in a small room, the wasted Co2 stays in the room and eventually gets recycled back into the intake of your tent, so you're not really wasting that much Co2. Keep in mind that this formula has 72 minutes of run time per 12/12 compared to 45 minutes with your proposed formula but I think you're going to need to exhaust more often than 3 times per 12/12 to maintain proper humidity and temp. Another option is to control your exhaust fan with a thermostat, which is a little cheaper but you won't have any idea how often it's running and they tend to be kind of janky, turning on for a couple of seconds every couple seconds, irritating.
