Thanks for your question, geeB! I have not tested the accuracy of the readings from the sensor, but after searching the web I found that the easiest solution would be to bring it outside once in a while. The other solutions would require a sensor that is calibrated or a room with a known value of CO2.
Maybe it is better to create a christmas star for the window (If we have to keep the christmas vibe)? Then it would be able to recalibrate because it is frequently being exposed to 400 ppm? I will do some tests in my working environment to see how big of a problem the recalibration is.