For multiplication, use the multimeter’s voltmeter (set to 20 DC volts) and measure the product of the voltage-divider potentiometers’ knobs. OPERATION: Two modes are used on the multimeter.

You mentioned your voltmeter runs at about 15 volts most of the time. My guess is the gauge reads a shade high and is reporting a healthy maximum system voltage of 14.8 volts, which is the norm ...