The resistance of the whole feedback circuit could be increased if there was a resistor in position ‘R’, but this would remove the intrinsic safety feature of the Zener and npn base-emitter clamping ...

To calculate the resistance of the series resistor ... Using properties of a series circuit, the sum of the voltages across the LED and resistor R must equal the supply voltage.

In other words, with the perfect setup, you could put a digital panel meter that reads the voltage in the circuit, and read the resistance directly. Zeroing it out would require more effort to get ...