Simple explanation of RNP
If you are RNP x then when the equipment in your aircraft indicates that you are exactly on the centreline - 95% of the time you will be x or less from the actual centreline.
submit 0.5, 1, 5, 10 or whatever the RNP figure you want to use is.
I think that you may be getting confused with the scaling of the display as one proceeds from enroute to terminal to approach and the different values of full scale deflection in each situation.
As an example if the best you could do is RNP 5 then regardless of what full scale deflection was, when the indication is on track i.e. no deflection then as I said above, 95% of the time you will be in this case +/-5nm of track.
RNP not just left/right and is not simply a circle. The shape of the area round the indicated position in which the aircraft could be will depend on the navigation systems used for the RNAV - GPS or DME DME or VOR DME or IRS or whatever. The RNP figure will be the worst case and for example in the case of IRS alone, there will be a time limit.
Regards,
DFC