I have no experience on the 747, just on the 737. On that bird we could change the scale of VRNP indicator on the ND on the progress page 4. Is that possible on the 747?
The FMC uses 400 feet as a default Vertical Required Navigation Performance (VRNP) value for oceanic, en route, and terminal phases of flight.
When required, VRNP values may be manually entered or displayed on RNP PROGRESS page 4/4. The FMC will accept manual entry of a VRNP value greater than the default value, but the VERIFY VERT RNP VALUE advisory message will be displayed in the scratchpad. Manual entries are cleared at flight completion.
We used it for example during RNP AR operation, although that was, of course, only approved in LNAV/VNAV.
The flight crew may enter 125 feet for vertical RNP. While there are no vertical RNP values published on the approach chart, the use of 125 feet will cause the NPS amber deviation exceedance alert to occur at 75 feet or slightly less deviation, since vertical ANP will be at least 50 feet at all times.
Of course, since it has been some time since i was on a Boeing, things might have changed in the meantime.