300E Duty Cycle baseline

[tilac1]

http://www.landiss.com/mixture.htm

According to this article, the author's battery voltage is 13.6 and to get a baseline; ignition on, engine not running, voltage between pins 2 and 3 on the X11 connector, I got 3.78 volts. Where is he getting the 13.6V from?

At idle I get ~6.7V and at 2500rpm I get ~5.0V

Since the duty cycle % formula uses 13.6 volts, I'm wondering where he gets it from. He mentions 14.0V at battery. Very confusing. Can someone shed some light?

1988 300E M103 60K miles