>that's 0.2% CO2, it that CO2 is the result of respiration, it means that about 0.2% O2 was consumed,

I dont know anything about human respiration, but I know a little about chemistry and theres no reason to assume this is true. Basic stoichiometry.

According to a random article on the internet[1], nominal co2 production is 80% of oxygen consumption.

Your point appears broadly correct, just wanted to point out some faulty reasoning that could lead to incorrect results in the future.

[1] https://societymechanicalventilation.org/wp-content/uploads/...

CO2 concentration doesn't start at zero, and by coincidence, if CO2 production is 80% of oxygen consumption, consuming 0.2% oxygen results in 0.16% CO2, add it to the base 0.04% and you get 0.2%.