BOAC References:- FAA AC 121-95 ‘Operational Landing Distances for Wet Runways’ quotes a factor of 0.96 between threshold speed and touchdown. (However, this is probably a ‘required’ min speed loss during test flights to determine the min landing distance, i.e. prevents ‘no flare landings’ being used).
NPA 14-2004 Operation on Contaminated Runways provides the following information which I believe originates from the certification assumptions/calculations in wet/dry operations.
7.4.2 Landing Air Distance - For contaminated surfaces, the airborne distance should be calculated by assuming that 7 seconds elapse between passing through the 50 ft screen height and touching down on the runway. In the absence of flight test data to substantiate a lower value, the touchdown speed should be assumed to be 93% of the threshold speed.
There is additional explanation in the comment pages.
Thus for a Vref of 100kts the speed loss is 7 kts, but this may be greater if test data shows otherwise.
IIRC several manufactures have used/assumed (at least in casual discussion) a 7 kt loss in determination of landing performance. Some auto land systems either have 7tks as a target speed loss, or it is the resultant of the flare design. Most autoland flares are representative of manual landings.
Again CAA data analyzing steep approach landings on one aircraft type indicated a 7kt loss in the flare; perhaps normal approach angles would result in a greater speed loss.
Overall a 7 kt loss appears to be an approximation to the certification requirements above, covering a range of large aircraft types and operating weights.