If you substitute the pnp transistor in your schematic with the equivalent model from [here][1], then you will see that the resistors R2 and R1 are forming a volatage divider. Let's call the potential on the probe V_probe. Using the node methode you can find that the base NP diode will have Vcc on the anode and Vcc\*R1/(R2+R1)+V_probe\*R2/(R2+R1) on the cathode. 

So my advice is to start with R1, which will determine the base current.
Base on that you can calculate the R2, so that the base diode will turn on, once the probe reaches the required potential. 


  [1]: https://wiki.analog.com/university/courses/electronics/text/chapter-8#bipolar_junction_transistor_large_signal_model