My alternator is charging in the 15v+ range at idle. I have zero add-ons. No speakers, no amps, no electric fuel pumps- this car is as close to factory as possible.
Look for voltage drop and downstream as Del suggested. Then with the engine running do it from the alt out which is your power source with the engine running.
Note the ammeter readings when you are doing this. There will be more voltage drop with more current.
The book shows the acceptable system Voltage range at various temperatures.
Now I’ve done some reading in previous posts about alternator issues and I read about stator replacements being wound with larger wiring to make one fit all situation from rebuilders. The factory style VR cannot handle the additional load and over charges.
The rotor creates the field. The stator reacts to it.
The current draw through the regulator is the field circuit.
If you want to measure the rotors resistance, measure slip ring to slip ring. The carbon brushes will mess up the reading.