CO2 and exhaust

I am currently supplimenting CO2 to my veggie room. The CO2 is dispursed as soon as the lights go on. while the lights are off the exhaust fan runs all night.
THe CO2 is dispursed rof 15 minutes and the exaust fan is left off for about 2 hours then back on for a 30 minutes right before the next CO2 cycle. this occurs roughly 5 times during the 12 hours. It's a closet sized space.
I don;t have a CO2 guage, My BPS is roughly 8. I'm looking for suggestions on anything I might be doing wrong or a better schedule for exhaust and CO2 dispersal.