An alternate and simpler auto calibrator would be a pair of wires with gaps that both go into the microphone like so:

Tone generator (speaker)_____ ____Mic in
Drip Sensor__________________ _|

Such that the spacing between the lower and upper gap is exactly a centimeter (or whatever unit the Peachy uses for calibration). Then run just saline through the dripper without any resin. The software won't start counting drips until the level reaches the first wire and have the software automatically stop the count when it hears the tone coming through the second wire. Suddenly, you know exactly how many drips are between the first and second gap!

It would probably be good to run multiple calibration measurements with it at different heights on the tank wall to be sure, so that you get a good average fill rate per drip.