That's the way that I do it too. My lamps are air-cooled with 2 blowers, and the first one pulls air from the room, which in turn pulls air in from outside. Since that's my only fresh air intake, I don't want to turn it off. So I just run my CO2 with that running too. I just calculated my CO2 run time, and doubled it. Instead of leaking gas for 5 minutes/hour, I leak it 5 minutes every half hour.
Not really scientific, just double the gas.
HTH