Measuring microphone Signal to Noise ratio (SNR) with A-weighting
IEC 801-21-20 Defines sound pressure as the RMS of of the instantaneous pressure, and as far as I know all meters use RMS.  The problem you have in your implementation is that you are normalizing the peak value of the active region but measuring the RMS of the inactive part. As you described, for sine waves the RMS is 3dB less than the peak.  You could take the Difference between the RMS in the active to the inactive regions.  Or you could assume that the active RMS is -3dB and use that instead.  Otherwise you should be good to go.