The locations of the chakras are practice and lineage specific. The subtle energy body descriptions are methods, not an objective reality. They differ a lot between lineages. While the "four finger widths below the nevel" is a very common focus point in many Nyingma practices, this is by no means universal. There are practices which utilize the navel itself, as well as ones which focus on a point further down. Besides, it is good to treat these things a little loosely, since the energy configuration regarding each practice is different for every individual. Generally, when the completion stage practices utilizing channels and currents and chakras are done, it is required to practice them, at least during the initial phases, in close supervision by a master. This is because some adjustments are usually made to accommodate for individual differences, and this sometimes will include the position of the chakras themselves.