Just received my new humidor

There is an open central chimney in the back of the cabinet or I should say the drawers do not go all the way to the back of the unit. I have a Hydra-LG Electronic Humidifier unit in the bottom. So far the humidity has been varying around 68 to 72 %. I am getting a discrepancy between the Hydra unit read out and my digital hydrometers.
I am going to test my digital hydrometers with a salt test and see which units are out and recalibrate. I guess the question is when should I put my cigars in?