1) If I had known how easy it is I would have jumped sooner. Really. All you do is print a target on a letter sized sheet of thick matte paper and tape it to the wall flat. Aim camera at center on same plane. Press go and change +/- setting in AF Fine Tune as directed. It spits out a number for your new setting.
2) Also love the test that checks all the focus points against each other. Nice to confirm I have a "good" D800!