You can use ambient air to do a 1-point calibration with the background CO2 level as your reference. You need a calibrated reference instrument to compare against, for example a GM70 hand-held meter with a calibrated GMP222 probe.
With the GMP231 and the reference instrument in the same space (for example, inside the incubator with the door open), allow the measurement to stabilize before comparing the readings. Try to provide as stable environment as you can during this time. Avoid working around the incubator during this time.