@andyp - Liveview and its' associated AF system uses the data at the image sensor, so Liveview focusing (contrast detect) is very accurate, (but slow). The normal AF system uses an AF module (Phase Detect) that is separate from the image sensor. Consequently manufacturing variability can result in a slight difference between what the AF system "sees" and what the sensor does. AF fine tuning seeks to determine the difference between what the sensor sees and what the AF fine tuning system sees and compensate. This compensation varies from lens to lens due to the manner in which manufacturing tolerances stack.
Unless you have a good test strip and target, and a solid tripod, AF fine tuning can be frustrating. Basically, in order to be successful at AF fine tuning you need to eliminate all other possible sources of blur in the image. This includes wind, mirror slap, tripod shake, shutter press induced vibration etc. A tripod is NOT optional.
Your focus target should be vertical and next to the middle of your inclined test gauge. Focus on the target in liveview, snap a few photos (to make sure they are all the same), and then do the same with standard AF. If the in-focus portion of the inclined test strip is not the same as the liveview sample, you may benefit from AF fine tuning. (+) numbers move the focus point away from the camera whereas (-) numbers move the focus towards the camera. Once you know which way you are out, you can add in adjustment until the two match. ... Sometimes I just take a bunch of pictures at different fine tuning settings and pick the best. Note: Be sure to defocus the lens between test images so that the AF system has to bring the lens back into focus.
If you search "LensAlign", you will see one of the tools available. You can do the same thing with a yardstick or ruler. Youtube has several tutorials as well. However ... If you are happy with your lens and the pictures you are taking, don't worry about it and have fun. If you find that for the way you are using your photos the focus seems to be off in a consistent fashion, AF fine tuning may be the key.
If you want the piece of mind, build a test rig (or buy one) and get the two AF systems to match, or get the viewfinder AF system (Phase Detect) to match your best manual focus obtained while zoomed in in liveview.
I am probably way too anal about this stuff, but I get some satisfaction out of knowing that my lenses are properly setup. Remember that AF fine tuning is not even available on many Nikon bodies; the reason for this is that the issue is subtle and many folks will never notice. However, with the D800 / 70-200 f/2.8 combo, AF fine tuning (or the lack thereof) may rear its head.
