IIRC , the system is 12 volts Right?
Power to the coil will be 12 Volts BEFORE the ballast resistor is connected and the coil is loaded (points open). You can have 1000 ballast resistors in line and open points, it will read 12 volts UNTIL the coil loads and the points close.
Coil loading occurs when the points are closed and power is applied to the coil. Then the resistance of the ballast drops it to 6 to 9 volts to preserve the life of the coil and the points.
When the coil is unloaded, (points open) the voltage jumps up to 12 volts BECAUSE the coil is not consuming Current. (No coil ground through the points)
To test while running is useless..you'll ONLY read peak average of the rise/Fall times of the points opening and closing divided by 6 or 8 a useless number somewhere between 10 and 12 volts..your meter (unless you have a VERY expensive meter with a VERY fast digital time base) will not have a fast enough time base to be accurate.