So Ferguson, if AF fine tuning produces a ski slope going up to the right and just about levels off before you hit +20, has a setting of +18 or +19 actually corrected the focus point...or just improved it?
I had that happen with my 50mm f/1.8 and when I put the 1.4x TC on the 70-200. The adjustment for the 70-200mm without the TC was zero.
Interestingly enough, the QoF with the TC on the 70-200mm was actually a little higher than before I put it on (VR was off for both) after I did the AF fine tuning with it on. On my D800, the QoF stayed about the same with and without TC. Both results are a great endorsement for the 1.4x TC on a 70-200mm f2.8 lens. (Not so with the TC on a 200-400mm f/4 with the D800. That is my next test with the D800E.)