Sounds like you're aiming for real time feedback rather than a calibration aide. For your idea I think static friction between the float and coil is going to be an issue, you might find you get jumps rather than a continuous change. For real time sensing then solid state wins it for me, I'm thinking guage pressure or capacitive ATM.

The idea of a calibration aide might be good though. For drip calibration you set a rate and let it count for a given rise in liqiud, then you can put in a drips per mm number. This may take some time and the more level you measure the better the result will be as you minimize errors. You could make a circuit board with a more accurate smaller gap which would take calibration time right down but hopefully maintain accuracy, this is as simple as a few transisistors and a cheap circuit board with some LEDs.