Y'All,
Tim and Mike are correct in that I was making an assumption as to the
accuracy of the GPS RNP-0.3 Minimum Aviation System Performance
Specifications (MASPS). As with practically all FAA
standards they only establish minimums. The .3 minimums need exist
only 95% of the time and they may not be linear but the diagrams
used to illustrate the RNPs use lises. (Figure 1-11 on page 1-13)

I am not a great believer in the validity of FAA minimums when my
flying life is at risk.
Gene Whitt