The resistance of the coil primary winding and the condition of the points will also factor into the amount of voltage the resistor drops. Dirty contacts in the ignition switch could also be a factor. It's only one link in a pretty complex chain of events between the battery and the ground the points provide. With the proper coil installed and a good set of points, the voltage at the downstream side of the ballast resistor should be around 9 to 9.5 when the engine is running.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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