fladd wrote:Is there a way to calculate, based on the measured volume, the specs of the SM58 and the specs of the interfaces, what the expected level in the DAW should be?
The sensitivity of the SM58 is published by Shure... but it's not actually relevant here as it is common to all your tests, and you are complaining about the different sensitivities of the two interfaces, not the mic in comparison to other mics.
However, 76.5dB SPL at the microphone should provide an electrical output of about -70dBu, and 88.5dB SPL would produce about -58dBu. Those signal levels will be raised by the analogue gain in the interface, and then converted into a digital signal according to the converter alignment as previously discussed.
Unfortunately, there is no way to calculate the digital signal level recorded in the DAW from a given sound pressure level reaching the mic from first principles because, as I explained much earlier, Focusrite don't state the internal alignment of the analogue/digital signal levels through the converters in the two interfaces.
What you can do, though, is measure the interface's overall conversion level -- as you have done -- by noting what digital level (in dBFS) is achieved for a certain analogue input signal level with the user gain control at a certain setting.
I'm still at a loss as to what exactly you're trying to achieve through all this, though.