He has already eliminated the possibility of a bad ballast resistor by measuring voltage at the:

Coil (+) terminal
Coil (-) terminal
Distributor side terminal

An open circuited ballast resistor would have stopped the voltage prior to reaching the upstream (+) side of the coil. It would have had battery voltage at that terminal during cranking, assuming the ballast resistor bypass circuit is intact and hasn't been disabled by someone who fantasizes that he's smarter than the engineers who designed the GM 12 volt ignition system. A burned out ballast results in an engine that runs as long as the starter switch is engaged, and stalls as soon as the switch is returned to the "run" position or the stomp switch is released, eliminating the resistor bypass.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!