Voltage drop across ballest resistor

Varies around 8-12V depending. Depends on system voltage, IE might sag at low idle, and depends on RPM. Also depends on what coil you have AKA how much current is drawn

Part of the reason it's impossible to figure is there are too many variables...............

System voltage changes

The ignition "run" line itself is subject to voltage drop in most cars

Depends on coil resistance AKA what coil

Depends on engine RPM

And, the ballast ITSELF changes resistance, which is WHY it's called a ballast. "Normal" design of a "ballast" situation is for the ballast device to attemp to maintain either current or voltage to a load. This means that as more current is tended to be drawn, and heats up the ballast, the heat / temperature/ resistance curve of the properties of the device will INCREASE the resistance, so that it will tend to then DECREASE the current drawn.

This is actually the "normal" behavior for something such as a standard light bulb. Any incandescent lamp draws a "surge" when cold, and when heated up to brilliance, the temp change in the filament REDUCES current flow.

This is actually a very big problem in large transmitting vacuum tubes, which must be protected during filament warm up.