For the i436, you can follow these instructions. This assumes that the sensitivity of your i436 is 6.7 mV/Pa. You should substitute the factory-calibrated sensitivity that you received with your i436.
Plug in the i436.
Tap the gear button in the toolbar at the top of the main screen in SoundMeter.
Then, tap on I/O Device Configuration to find the Input Options menu.
Make sure the Input Device is "Headset Microphone".
Make sure Device Units is set to "V".
Then, tap on Input Channels and make sure that All Channels units is set to "Pa".
Under CH1, tap Calibrate, and in the Input Sensitivity text box you can enter "6.7m" or "0.0067" to make sure that your input sensitivity is set to 6.7 mV/Pa.
If the Input Sensitivity text box shows units other than mV/Pa, then you must have missed one of the earlier steps.