This bottom rail controversy is all dependent on what mounting hardware is used for the camera on the upper rail.
53-57mm us fine if you are using CP1 or CP2 however if you are using quick mounts the situation changes dramatically because those types of mounts are much thicker.
I am using a RRS L bracket on my 5D MKII and mounting that to the NN (ASQRS2) arca style quick release which is substantially thicker when put together.
The end result is that 67mm is the correct setting for the lower rail on my NN5L with my mounting system.
Hope this helps some solve the lower rail setting problem
Croce