It is a bit more complicated than 10 + 10 + fudge factor, have a look at a cogent explanation -
Required Navigation Performance - Wikipedia, the free encyclopedia
RNP10 means the nav error of the aircraft will not exceed 10 NM for at least 95% of the time. That is not enuf tolerance for ATC standards, which require 99.something or other%. To get that level of containment you need a circular error of position of 25 NM, put your RNP10 aeroplanes 50 NM apart and they are separated. Don't need a buffer.
There are other same direction standards that let you get closer than that, and radar is better.