First and foremost, make sure that it really back focuses. Most advice on testing for this is just dead wrong, because it doesn't account for what the autofocus sensors actually see and do. Nor does it account for field shift, an attribute of many fast primes.
Putting an autofocus sensor on a test target that's at an angle won't get you the right results. That's because the autofocus sensor is really an array of sensors and something has to sort of "vote" on where the subject is. If more of the array is on the background than the foreground, guess where the camera might focus?
Nikon sort of mentions this in their manuals when they talk about use of wide angle lenses shooting small subjects with busy backgrounds using a wide angle lens. What they're trying to say is that more of the autofocus sensor is likely to see the busy background than the small foreground subject in such cases, and thus the camera will focus on the background.
I recommend that you use a product such as LensAlign. If you look at the design of the LensAlign product, for instance, it does things right: you focus on a target that is absolutely parallel to the sensor, thus eradicating the potential for the autofocus sensor array to pick up something at a different distance. Then you READ the answer on the diagonal chart that's attached. They also have optional FocusTune software that will do that interpretation work for you. (Other solutions exist, but LensAlign was really the first one that did it right, and is simple enough that anyone should be able to use it correctly. Disclosure: LensAlign has provided samples of their products to me without charge for testing.)
If you already have a LensAlign you already know the rest of the answer to the question: use the LensAlign tool and the camera's built-in AF Fine Tune function to correct the focus. Low end consumer Nikon DSLRs don't have AF Fine Tune (e.g. D3xxx and D5xxx); only the high-end consumer, prosumer, and pro DSLRs do (i.e. D7100, D7200, D300s, D500, D600/D610, D750, D800/D800E/D810, D4/D4s, and D5).
I'd add a bit of a caveat to the preceding: if you use multiple bodies or zoom lenses, AF Fine Tune doesn't necessarily fix your problem. Moreover, lens/body combinations that are far out of alignment (>10 on the AF Fine Tune adjustment) probably ought to be looked at by Nikon. In my experience, something that needs a compensation of 15-20 in AF Fine Tune means that something is near manufacturing tolerances (and can often be beyond correction if another part of the body/lens equation is built to what amounts to an additional compensation), and only Nikon can really fix that permanently. Unfortunately, the feedback I get from people using Nikon repair service to adjust AF Fine Tune values is highly negative for the most part: it often takes two or more repairs to get things "better," and rarely do you end up with products that are at 0.
