For accurate reporting you need to relate the actual stopping distance with the expected distance (ICAO definitions below), but as both depend on max brake this is generally impracticable.
Thus, most pilot reports are subjective (and very variable). The sensed deceleration relates to the amount of brake applied and the friction of the surface; again quantities which are hard to define. Also, remember that deceleration (braking effectiveness) may vary along the runway, particularly where there are rubber deposits.
IMHO pilots only report when they have been surprised by the conditions not meeting with their expectations, this usually results in a ‘medium’ call, whereas in reality it probably is ‘poor’.
If asked to report, the ICAO guidance can help if ‘expected distance’ (a norm based on experience) is substituted for the scheduled distance; this provides some correlation with the conditions when landing on non limiting runways where max braking is not used. The more limiting the runway length is then the closer ‘expected’ and ‘scheduled’ become, and also more braking is used.
When deciding to land based on a pilot report of braking conditions, don’t forget that “you are betting the safety of your ‘butt’ of the feeling in some other pilots ‘butt’.”
ICAO reporting:
Good:- Aircraft can expect to land comfortably within the scheduled distance, where this is “wet” distance, without undue directional control problems.
Medium:- Aircraft are likely to use all the “wet” scheduled distance including the safety factor part of the distance, and may run even further. Directional control might be impaired.
Poor:- Aircraft can expect to run for up to the full “very wet” or aquaplaning distance where this too is scheduled. Directional control will also be poor.