I have an X-Rite 301 B&W Transmission densitometer and yes I do have a calibration tablet. It does calibrate to the desired reading after I have powered it up and nulled it properly. I turn it off when I'm done, sometimes the next time I go to use it, power it up, null it, then insert the calibration tablet, it seems to always have to be adjusted again, i.e., it is more than .02D units off. I was just wandering if this has to do with the power and perhaps I should turn the unit on let it remain on for a good while before use.