[RichardJ]

The charging circuit has nothing to do with the Ammeter circuit.

That may sound weird or even wrong, but there is only one wire segment that the two circuits have in common. That one wire segment serves double duty. For the Ammeter the wire is a shunt. For the charging circuit, that wire is the charging wire for the Alt/Battery.

You didn't have to add a resister because it is already included inside the original dash wiring harness. The resistance needed for a SI conversion is the same resistance used for the original 10DN external Reg Alternator.

The resistance used is a resistor wire and is part of the wiring harness from the bulkhead connector up to the ignition switch.

The resistance wire is the same resistance wire used even if there was no idiot light on the dash. It's there.

The resistance measures about 10 ohms.

The resistance needed for the CS Alternators is higher and that is why you will see posts about adding a resistor for those installations.

Do not confuse the resistance wire used for the Alternator Exciter circuit and the resistance wire used for the points ignition circuit.
Those two circuits are totally independent of each other.