How accurate reading can I get with SoundMeter after calibration?
If you calibrate at 1 kHz, then you should easily be able to achieve accuracy within 0.1 dB at 1 kHz. You should be aware of the current high-pass filtering applied the iPhone's built-in microphone. This issue is discussed on our blog.
If you need true class compliance, then you'll need a dedicated sound level meter build for that purpose. MicW makes a microphone that plugs into the headset jack that they claim conforms to standards for Type 2 sound level meters. However, the iPhone's headset input also passes through the high-pass filter.
We're hopeful that Apple will provide developers with a means to bypass the mic input filter, but we won't know for sure unless/until it happens.