A nuclear magnetic resonance (NMR) logging tool assembly method that employs rotational indexing to optimize the sensing volume. At least some embodiments include establishing an initial arrangement of the permanent magnets and marking each magnet to indicate their relative rotational orientations in the initial arrangement. Thereafter a series of magnetic field measurements and individual magnet rotations are performed to improve the uniformity of the magnetic field in the sensing volume. Once a satisfactory arrangement has been found, the magnets may be secured together and an antenna array installed along with the electronics for performing relaxation time measurements and providing logs of formation properties that can be derived therefrom, such as porosity, permeability, density, rock type, fluid type, etc. The tool may be packaged as a wireline sonde, a tubing-conveyed logging tool, or a logging while drilling (LWD) tool.