Well, there are a couple ways I've seen lipo pwms run, and it looks like this:
The way you have it would be right IF the potentiometer (cant spell) actually limits the voltage by only opening the mosfet partially, to allow a smaller current to flow through it, hence, lower voltage.
But the more common way I've seen it done, and probably the way you have in yours is that the actual voltage is at full power, but you control the duty cycle(the time the power is applied to the atomizer) So, 30% duty cycle would mean you are vaping 16.8 volts across .38 ohms for a total of 742.736842 watts, But only 30% of the time. You could say 30% of 742 watts is 222.6 watts, but it really doesn't work that way. It might feel that way, I don't know because I never tried it
These pwms pulse at an adjustable rate(by the potentiometer). so by adjusting the pot, you're just supplying that poer more or less of the time. But it's the same power regardless.
If your device has a volt meter built in that changes with the pot, you could have it the way you described in your calculations, but still highly unlikely. But those voltage meters measure RMS, not really an instant reading, so you can have those read some baffling numbers when you chuck clouds. Thats why a lot of box builders have it to measure battery voltage only(and because you can throw a 3s lipo in it and still know when your battery needs charging)
I hope some of this helps. I had my first PWM back in 2011, and I got it all wrong explaining it, but never took it dow. But just for kicks, heres the video: