Unfortunately, the i437L presents itself as if it were a standard headset jack. This means that apps like SoundMeter cannot automatically distinguish between an i437L and a standard headset jack.
Your approach is correct (i.e. Device Units = FS, Channel Units = Pa, Gain = Mid). We do not know the precise input gain used by MicW when testing the sensitivity of the i437L, but using the Mid gain setting in SoundMeter seems to work well.
It's too bad MicW didn't take advantage of the opportunity to produce a digital microphone that presented itself uniquely to iOS. That would have allowed automatic loading of sensitivity information, similar to what SignalScope does with the Digiducer digital accelerometers.