On a Nikon, (AF fine-tune) is linked to the lense’s s/n in the built in CPU, usually you assign the last 2 digits of the lens (from the CPU) to the newly tuned value.
The issue you have with non Nikon lenses (eg Lensbaby, with no CPU) and back focus (which is typical for most Nikon lens/body combo, so go to the minus side) requires a (Default) value adjustment which mean GLOBAL FOR ALL lenses and saved (AF fine-tune) values. That means ALL LENSES will be +/- from what ever you may have done to specific Nikon (AF fine-tune) values.
The reason you need to change the (Default), is that the Nikon body cannot assign a specific +/- value to an item with no CPU. Nor can it get the data you assigned in the (No-CPU lens data) = that is only used for metering. So you cannot assign a non-CPU lens number to the (AF-fine tune) menu.
So, in your case, I would change the (Default) to a (-) or (+) value that you can ascertain using a ruler at a less than ~45 degree angle, and focus in the middle using the ( > o < ) green indicator in the lower left corner of the viewfinder. Keep adjusting the (AF fine-tune) until you get a sharp result for that lens. Make a note of the selected value, and use that corrected value in the (Default) each time you use that specific lens. NOW, YOU MUST RETURN the (Default) value to (0) when finished, and going to use another lens. A good tool, is Focus Tune by Michael Tapes Design. You will discover that all your Nikon lenses focus at different +/-. The (Default) setting rarely, if ever, needs to be changed, unless the AF system is out of whack, and you are far, on a trip, from a Nikon Repair center. Good Luck, hope this helps, and logically clear. Oh!and make sure you turn on the (AF fine-tune (On/Off) to enable the changes… return to (Off) when finished using that lens.