I wouldn't get too precious over that calibration, that resistor is sensing the current through the feedback loop thermistor of an audio oscillator, which isn't quite the same thing as the output level. AND the output goes through a different type of thermistor and a twiddle pot before it goes to the transformer and the valve under test. This is not high precision stuff
An audio millivoltmeter with a few k ohms or greater input impedance would be fine.
You have to look at this circuit to fully appreciate the benefits in the Sussex design
David