Nikon technical bulletin on using the D800 recommends use of Live view for critical Manual Focus tasks. the live view has a feature that allows you to zoom in to a very small portion of the image to assist with highly accurate MF setting.
note that on some lenses, the sensitivity of the MF ring on the camera is poor. the entire focus range may be crammed into a relatively small arc of rotation so it can be difficult. The lenses with combined Af/MF clutch and the macro lenses tend to have a wider range of movement allowing critical focus adjustment to made more easily.