Greetings
The B777 is equipped with the NPS Navigation Performance System, along with a Navigation Performance scale for lateral, and vertical RNP, and a directional Indicator.
the full scale represents the RNP value regardless of its value, i.e., RNP 1, RNP4, RNP 0.3, so the RNP value doesnot not change the scale width ( this is why I refered to the diameter of the tube, and not to its Radius)
In the middle of the scale, you have a mark (I cannot find the word) now everytime the pointer is in the middle it is showing you that you exactly on track, if the pointer goes to the full deflection to the left, it means that you are on the right of the track, with a Xtrack error equal to half of the RNP value, the ANP will turn amber, and the pointer will flash, showing you that you have to fly left to regain the published track, and vice versa.
The AMC 20.27 states that for an RNP approach the maximum deviation should not exceed half of the published RNP, for that particular segmentg,i.e., IAF to FAF RNP1 so max dev is 0.5, FAF to MAP RNP 0.3 so max dev is 0.15, and from MAP and GoAround segment, RNP1 so max dev is 0.5.
FAA.GOV website has all the literature, have look.
Regards