Please verify my calculation.

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I'm trying to figure the current draw of a car amplifier. Currently, I multiply the total watts RMS by 2 for amplifier inefficiency, then divide by 13.8 volts. Since the average music signal requires about 1/3 of the average power in a test tone, I divide by 3. I then calculate 40% of the amperage of the stock alternator to account for running the vehicle (lights, heater fans, etc.) and add this to my previous calculation to come up with the total approximate average current draw of the audio system and what's need to run the vehicle. Does this sound right?