<<Zoom lenses perform differently at different focal lengths. I saw test results recently where the greatest error on a 24-70 lens was around 50mm - and it was in the opposite direction of the errors at 24mm and 70mm. >>
I've been following this post with interest and also have tested numerous lenses via the LensAlign and FocusTune, both prime and zoom. I got myself in a real mess as I only undertook the testing as I was interested in the concept. In the end I abandoned it in favour of testing my zooms against a flat target. My conclusion is that attempting to calibrate zooms is fraught with difficulties unless a single focal length is selected I.e. treated as a prime. My 24-70 and 70-200mm AF-fine tune values varied enormously between the widest and longest focal lengths.
With the primes I did have success and was pleased that the required fine tune setting was minimal (+-0 to 5). In fact in real world operation I noticed no difference in IQ unless really looking very hard.
In the end I suppose I am lucky and I have abandoned fine tuning and reset all values to zero. I feel if I hadn't I would have imposed a problem that wasn't there in the first place. I do feel the pain of those having problems with their D800's and hope they are resolved quickly.