Wet runway - 1.15 factor

[tribo]

alf5071h, that is the "correct" 15%.
I have for some time tried to find out how it came into being. I know that the ICAO Airworthiness Committee, Flight Working Group, int the mid 70's were developing guidance material on performance information for wet landing surface definition. From a working paper (AIR C-WP/530, Attachment):

In response to a recent FAA analysis of test of a Caravelle Jet transport and a DBV, and from their own analysis of a Mu-Meter that was tested at the same time the State of France concluded:

1. ....."it seems that the Mu-Meter is too much sensitive at the runway texture to predict the aircraft landing distance on any runways."
2. "The FFA concluding remarks which establishes 'that 15% factor contained in the FAA proposed landing rule change appears to be justified in view of all the tests completed to date' is correct in our point of view." (FAA method based on DBV). (DBV = Diagonal Braked Vehicle)

I suspect that I might be on the right track.
In an other working paper from the ICAO Airworthiness Committee (AIR C-WP/551 dated 29/1/76) there is a figure of calculated stopping distance vs. test stopping distance using SDR MuDRY/MUWET relationships. Tested aircrafts are L-1011, B-737, B-727, DC-9 and Caravelle. In this figure a '15% line' is drawn.
In the working paper there is a reference to a report FAA-FS-160-74-2, August, 1974 with respect to aircraft data reduction. Do you know this report?