safetypee...
The 2.5 NM primary area, under Pans Ops criteria, to which you refer is based on the route-sum-square value of several parameters. One of those parameters is "flight technical tolerance", which is allowance for the pilot's ability to maintain the required track.
Other considerations are applied, however, so it isn't a simple matter of saying that DME arc tracking tolerance is 2.5 NM.