I should imagine, calibrating the printer for various audio inputs could be challenging. Presumably an amplifier with AGC must be used, or the size of your print would depend on where you set your volume control. Even with an automatic gain control amplifier, there could be issues at low volume and high volume. At high volume, there is risk of clipping and hence inaccurate x and y due to clipping distortion. At low volume, the same applies (detail could be lost). I presume the Peachy must have some LED style VU meter, so that you turn the volume up until the LED goes green?? Maybe !!!!

You could achieve calibration manually by feeding Peachy with a calibration signal which creates a laser pattern against a calibration measure floating in the tank. Then adjust the volume until the laser just strikes the points on the measure.