I'm definitely not one that believes that non-zero means bad, provided I have headroom to hit the right value.
>- How big is the difference in between the central and the >outer sensor? I mean, what is the difference in between the >optimal AF Fine Tune for the sensors?
The only one I checked the left/right points was the 85/1.4G. The number above was an average. The actual numbers were:
Left = + 2 Center = + 18 Right = +5
On paper that sounds horrible, but the shape of the curve for the left and right were pretty flat, so picking an average of +13 yielded an acceptable compromise. And in fact I couldn't really tell a difference in left and right by eye.
I believe left/right/center should be nearly identical and they are not. But it was "good enough" that on all my other lenses I could not tell the difference by eye in real images. Only on the 85/1.4 is it an issue, and only very slightly. I have not tried comparing inside and outside (but I use the tuned lenses both places without any apparent difference).
If you are expecting to be able to measure exactly the same value for left/right/center, my GUESS is you will never get there. I think the tolerance is beyond Nikon. But I think they CAN get it close enough that real images are OK. It doesn't sound like they did for you.