DotTune is a method to set AF fine tune for a given lens on a given camera. The technique was first proposed by Horshack on DPReview in this post and at Fred Miranda for Canon and Nikon. The technique uses the green focus dot in the viewfinder to determine the correct focus, based on changing AF fine tune values. This works because the camera (Nikon and Canon) use the AF fine tune value as an adjustment before focusing.
*Using a greater target distance tends to work better than a close distance. The adjustment might vary by distance, but longer distances adjust better to shorter distances than the converse.
** Nikon cameras use a less sensitive focus indication when the focus is set to manual. To get around this limitation set the focus activation to the rear AF-ON button and then use the shutter half press to check the focus dot. This will activate the focus dot without focusing the lens.