I've found that just having a tilted window is enough to make a difference. Then it's always <800ppm. I do monitor C02 levels in the office, and noticed that I

I've found that just having a tilted window is enough to make a difference. Then it's always <800ppm. I do monitor C02 levels in the office, and noticed that I never check the value if it's <600ppm, and that anything >800ppm makes me unproductive.