The current flow and the voltage drop across the resiistor is a function of the total resistance in the circuit. The coil designed for 12 volts will have more resistance than the 6V coil, so the total will be more. With the points closed, some of the voltage will be consumed forcing the current through the resistor, and the rest will be used by the coil. With the points open, you'll see 12 volts all the way to the insulated point, because no current is flowing. The best way to get an accurate measurement of voltage drop across the resistor is to measure voltages with the engine running. You should get close to charging system voltage upstream of the resistor, maybe 14.5V or so, and 9.5-11 volts at the coil + terminal.
Jerry
