Quote: The sensitivity on those gauges vary from year to year and make and model, some of needles in my mid 80's trucks wouldn't even move but I had a 1968 Dart that the needle was always flopping all over the place especially with a turn signal on
I had my gauges professionally done by Instrument Specialties but I have found that the gauge ranges from around -40amps (discharge) to 40 amps (charge). Everything in between should be pretty linear.
If your needle is bouncing when the blinker is on, that means your alternator isn't keeping up and you have steep charge/discharge cycle every time the blinker blinks. I'd be willing to bet if you revved the motor a bit with the blinker on, the bouncing needle would go away.
