I've started coding the python that needs to run on the ESP to drive the hardware part. It requires the PWM for the HV, and an interrupt handler to listen for and act on discharges in the tube.
It will live on github.
It features a calibrate routine, which will try to produce the plot as described in the details section of this project. This eliminates the need for HV measurement equipment, but still enables the end user to set the correct PWM duty for the correct HV required by the geiger tube.
I'm using mpfshell to upload the two scripts to my ESP.