As I see it, the major problem is that we're not told about the set up in the aerodrome booklets.
OK, so we know what the PAPI 'on slope' angle is set at, but we're not told where the PAPIs are located in relation to the glideslope aerial. Many airfileds(but which ones??) have the PAPIs set up 120m upwind of the glideslope aerial - so you will/ you must get at least three reds if you maintain the ILS glideslope.
The aiming point reigns supreme.
But it would be nice to be told.
However, aren't visual slope aids purely calibrated for Cat I minima, therefore not to be relied upon below approx 200R?