Bill.........NO, unfortunately I don't have enny write-ups to convert to 12-volts. Do a search for JMOR who has published several good pictorial drawings to convert to 12-volts.
You ask......."What voltage reading should you get from the output of the dropping resistor to ground?".......would you believe 1/2 the applied battery voltage? That means about 6-volts for a 12-volt battery. When the points are CLOSED! When the points are OPEN, you will read the 12-volt battery volts. Ittza LAW, Kirchhoff's Law. ........Dell