Not charging with headlights on

My car is a 1970 Duster with a 1965 273 engine. I have the Pertronix Flame Thrower Coil and the Ignitor distributer and a bypassed ballast resistor. Everything has worked fine for a year this way.

I noticed while driving home in the dark that my headlights where dim and that my alternator ammeter showed the car was not charging, see picture. I turned the headlights of and it went back up to the middle. Because It was dark I had to have the headlights on so I went straight home. Getting close to home the car started to stutter, I turned headlights of and it stopped.

I have very little knowledge of electrical issues. I re-charged the battery and checked it with a multimeter. With the engine in idle without the headlights on it read 12.20 and while reving it quite hard it never went up to more then 12.27. With headlights on it read 12.02 on idle and did not went up at all while reving.

My friend had an spare alternator (not sure of the condition of it to be true), so I tried that and it was the same.

I’d really appreciate some pointers on how to use the multimeter to look for the issue before buying a new voltage regulator or alternator. Tried to explain best I can in english aswell :)