I'm not totally sure if this is correct, but a motor rated at 12 amp @ 115 volts should use 1380 VA (Watts). If it's the only device on the circuit and is using 15 amps and we divide 1380VA by 15A and get 92V. The voltage loss would be your unloaded voltage minus 92. If the unloaded voltage is 110 there's an 18v loss, and a 28v loss @ 120v.
I don't think the voltage loss is necessary to know, we already know the motor is using more current than it is rated for, and over-current is what burns up wiring such as the motor windings. Circuit breakers are designed to protect the house wiring from catching fire, not the devices connected. I personally think 15 amps is more than adequate on a 12 amp motor by itself and I would keep it on 15 amp breaker to limit the over-current to 25% more instead of 66% more on a 20 amp breaker. I would go buy a better extension cord and put it back on the 15 amp breaker until the permanent line can be ran.