radovantz:
In my experience, i found that 0 is the default volume set by manufacturer so that every channel should produce the same loudness. These volume however are useful to balance all speakers only when they are positioned at different distances. Test tone might be helpful, but, I wonder why does your centre channel need additional number to balance with others.
I would try to connect that centre to L/R, just to check whether the speaker is really quiet. Otherwise, i think your setup is right
It obviously depends on where your speakers are situated. My receiver allows each speaker to be set from -20 to +20. Mine are all different (my rear speakers are smaller than the rest and are -5 and the front left and rights are -8 with the centre at -3. The sub is -15. When I ran the auto calibrator, it set the sub at -20 and the others all around -10. It's up to the listener though to set it how they want. I would advise however, not having them in the plusses and just turn your main volume up more.