I disagree. I feel a very steady and progressive deterioration starting at 600 ppm. It becomes significant at 800 ppm. The studies back up the latter threshold.

Unfortunately it will be hard for you to know how much of that effect is placebo. Unless you tested this with some kind of double-blind setup.

You would not notice a difference if you weren’t checking the CO2 ppm. You primed your brain to ‘feel’ the effects of higher CO2 by reading a study and are experiencing the nocebo effect.

If it makes you feel better I don’t see a problem with it.