What's a reasonable fine tune for most of your lenses?
Let's say, for example, that you have six pro lenses, and five of them require -15 and one requires -5.
Pretty clearly that suggests that your camera body is probably running at about -15, and that's the camera body error level, and one lens is likely at +10.
Is -15 a reasonable adjustment for a $3,000 camera?
How about -20??
IOW, is a D800 body at the maximum correction of -20 acceptable? Or does it go in to Nikon for adjustment?
How about a camera body running at -15?
Do you think Nikon would say, "Hey, -20 is within specs! We won't work on it!"
Ditto for -15??
We all recognize that autofocus is a very hairy electro-optical-mechanical function, and we can't expect perfection. In fact, we can be glad we can get what is available. But the big question is, "What's reasonable for $3,000?" (Heh! 5 years ago the question would have been, "What's reasonable for $5,000?" Or "$10,000.")
(We also realize that a user could have 5 bad lenses and a perfect D800 body. So all this is a bit hypothetical, however unlikely. Looking for "reasonableness" here.)