Kept the windows in the bedroom closed last night. Netatmo shows CO₂ creeping up throughout the night, until I open the window in the morning. Subjective air quality was poor, but not headache-inducing bad.
Meanwhile, the Flow maxes out at an AQI of 13 for VOC (anything less than 20 is considered “good”).
So while changes to VOC levels do roughly mirror CO₂, VOC levels alone aren’t sufficient to help me decide on how much ventilation a room needs.