Technically, any level of CO2 causes detriment. Even our ambient level is higher than some decades ago and apparently it is possible to measure the effect on us (mostly related to nervous system).
Personally, I can't discern any difference below 600 any way I tried to measure it.
I carry my Aranet4 with me and in some apartments of my friends and family it is normal for CO2 to not go below 2000ppm in winter. So you are doing pretty well.
I think this is incorrect. You may be thinking about PM2.5 levels of which there are no safe level, any amount is harmful. Similar for CO. But a certain non-zero amount of CO2 is actually required.
Personally, I can't discern any difference below 600 any way I tried to measure it.
I carry my Aranet4 with me and in some apartments of my friends and family it is normal for CO2 to not go below 2000ppm in winter. So you are doing pretty well.