this is beyond veritas and REW, just waste a night googling about loudness measurements standards and what exists to estimate perceived loudness(which has little to do with actual loudness). in the end it will be your preference and not much else. you've set REW for 1khz tone, that's the only place so far where you know for a fact that the values are correct. the microphone doesn't have a flat response(most professional microphones would come with an individual calibration, making it all much easier). the IEM also won't be flat, the er4 for example isn't the loudest at 1khz. so we don't really have what it takes to start worrying about a few db or take what we measure at face value. this is low fi measurement, we have to go with what we can do for the money. in case it's not clear, Zpeak stays at the highest peak value it gets, just like LAF max doesn't go down over time. it's a little soon to go all paranoid on values ^_^. and Z is no weighting so if you get some good bass(where often the max amplitudes will be in music), plus your IEM doesn't roll off like crazy in the low end, then it will show.