In order for the thing to be even close it needs to have lots of air flow in the room , from the floor up. As co2 falls the meter reciever must be located mid room about thickes part of the blooms. Now thick air cooler will have a different reading cause the air is thicker.
Normal out side is around ?450 ? If the room is well sealed if you do not breath it can be around 1400, thats a 13 x12 x 8 ft.
Also what is the humidity in the room , that may be one more factor.
NOTE check the span on the meter and make sure conection are clean.
Look into getting a Dataq usb to read the output over the total cycle. They are about 50 bucks, and are well worth it. Use some speaker wire to get your reading.