Graeme - just to reiterate, in theory, you want to calibrate the sound level from each speaker so that the SPL meter indicates the same volume for each speaker.
In actual practice, however, many listeners calibrate their center speaker 2-4 dB higher than the rest (i.e. turn it up until the SPL measures 77-79 dB if the rest of the speakers are at 75). This is done because it helps to more easily hear movie soundtrack dialogue. Additionally, as others have noted, some of us like to run our subs a bit louder than the rest as well (myself included). I calibrate my subs to +3 dB than the rest of the system (again, around 78 dB).
Experiment to see what you like. These are just guidelines, but they should get you close.
Jason
