To calibrate a sand-moisture meter, I requested the customer to make several plastic bags with wet sand. Each sealed bag was filled with sand of a known moisture (per cent water weight). In the calibration I obtained several sets of readings, then plotted a curve (X/Y plot) of calibration points, plus error bars.
If you calibrate an air-moisture meter, take a calibrated one and generate a column of air passing through both sensors. Record integer readings of the calibrated meter, along with readings of your meter. You might run the process several times and adjust the maximum reading of your meter for the maximum desired moisture.