What type of meter? Not sure what you mean by @20. I ask because most digital meters will give automatically scale to show ohms, or megaohms; while most analog versions there is a scale to set and then read. With the scales sometimes you have to multiply.
As far as the VR in your '75, no you can't test it that way.
The 'I' terminal checks the voltage in the run circuit. Depending on the voltage level, it powers transistors which control how much current can run through the alternator's rotor. Basically it controls the other terminal's connection to ground, opening and closing it as needed.
