Yamaha CS80

Hi all, I have a question regarding the CS80 tuning. I have followed the service manual instructions, i.e set upper octave pitch, then scale with the lower octave using V3 and V2 and then repeat until both are within 1 cent. This works fine for some voices, but for others, having done that I then find the middle C is around 6/7 cents out. If I tune middle C then the lower octaves are around -15 cents. Should I be able to get it all to within 1 cent as it says? I could spend the rest of my life doing this!!