Let's step back a square or two. Can you measure the dark blue "ignition run" wire, what you want to do is check for harness voltage drop
Turn the key to "run" engine off. Hook one probe of your meter to battery positive, the other probe to an "ignition run" connection, either at the ballast, or the blue alternator field wire.
You are hoping for a VERY low reading, below .3V (three tenths of a volt) If more, you have a drop somewhere in the ignition supply harness, either the bulkhead connector, the ignition switch connector or the switch, or the ammeter circuit.
What this voltage drop does, is the regulator "reads" this voltage, and if it's low, the regulator jacks the system voltage up until that line comes up to the regulator setpoint. If the regulator is properly operating at around 14V, and you have (as my 67 Dart) let's say a 1 volt drop, the regulator ADDS that to it's setpoint, so the battery sees 15V
I've got lost, here, did you check voltage for regulator ground?
With system charging and running, stick one probe on the battery NEG post, and the other on the regulator case. AGAIN, as above you want a low reading, the lower the better, Over .2V or so means a poor ground between regulator, body, block, and battery.
As usual, Big Dan has not bothered to explain himself. I suspect he is intimating at this very problem, IE a bad ignition switch