Hi Chips, the -12dB is the signal level you are playing through your system, The SPL measurement is from your calibrated UMIK1. If you turn up the signal your measured SPL will be louder. I think this just lets you see the SPL of each speaker relative to each other. After the audyssey I think they should all be the same in the main listening position (where the audyssey microphone position #1 was). I'm not clear what you are trying to calibrate after the audyssey, and don't think I know enough to help. It might be a good idea to explain what you are trying to do in a bit more detail so more knowledgeable people can help. Pete.
Edit: 75dB is the reference level Audyssey is using for its calibration, ie the SPL measured at its microphone during its test sounds (and what you would measure with an SPL meter if you measured it at the same time). For the audyssey calibration it controls the signal level and amplifier level to get the readings it wants at its microphone. When you do it manually you control the signal level and the amplifier level to get whatever SPL you want at your UMIK1. Looking at the 'Checking Levels' section in the REW manual it says the following:
"Press the Check Levels... button and follow the instructions on screen. The test signal defaults to an RMS Level of -12 dBFS. If connected to an AV processor, start with the volume fairly low and increase it until the meter is reading around 75 dB. The exact level is not critical."
So from that I think you do leave the generator at the default of -12 and adjust the amplifier volume until the SPL at your UMIK1 is about 75dB. Where did you read about setting the signal generator to -30 ?
Pete.