Phase with delay is a very tricky issue. If you don’t specify the delay precisely, then you are left with meaningless phase that is unwrapping quickly that tells you nothing
Consider a 1 kHz sine with a period of 1 mS. If you know the system has precisely a 200.0 mS delay, then you can determine the shift of the 1 kHz. But how do you know if the system delay 199.5 mS with 180 degrees of shift at 1 kHz or 200 mS with 0 degrees at 1 kHz? If you don’t get it precisely right, then your phase measurement across the band is meaningless.
Now, imagine a system with precisely 192.377 mS of delay (unknown to you) and 16 degrees of phase shift at 1 kHz. How can hope to learn that? If you ballpark the delay at 192.4 mS based on say, a scope measurement, then the 23 uS of error is appears as ~90 degrees of phase error at 10 kHz.
It’s very tricky.
For all phase measurements, you need a reference point. The reference point for the QA401 phase measurements is the output before the DAC. What would make sense is a two-channel approach, where the right channel would be used as the reference and the left channel as the input.
A third approach involves centering the recovered impulse response in the window. That can be seen in “Amp FrequencyResponseChirp” plugin (tick the phase box). In order for that work well, you need a reasonably flat passband. But you will still be thwarted by the sharp filters in the ADC and DAC–you can see this in loopback. Remember, a low pass filter will start showing phase changes a decade below your cutoff frequency. In the plot below, this is a chirp with phase plotted. In dark blue you can see the ADC/DAC filters roll off very 50 kHz or so, but you can also see the phase has changed 45 degrees at 5 kHz. A measurement that relied on the right channel as the reference would nullify this.
So, I think your goal is correct, but I think the better way to achieve would be to add a checkbox to the plugin that allowed you specify right-channel as the reference input.