There isn't, to my knowledge, a calibration method to basic tank level indicator. Waste tank readings are problematic due to the stuff in the water that can affect the sensors. Flushing after dumping helps but also check all the sensor connections and the ground point for possible corrosion which could cause additional problems.
I flush the tanks after each outing (if possible) and have found the sensors for my tanks are for the most part accurate although I don't think the installer correctly positioned the sensors. I seem to hit the 1/3 level of the 30 gallon black and 40 gallon gray tanks the first time anything goes down the drain and 2/3 not long after. Hitting full will take a few days.