UK CAA AIC 11/98 “Landing performance of large transport aeroplanes” provides an excellent briefing on the margins assumed for airspeed etc at the threshold, noting that the margins are for safety reasons to accommodate error and not for use in planned operations. Also note that any margin may not protect against error in more than one variable i.e. fast and a long landing.
The aircraft manufacturer may recommend additions to the calculated threshold speed for wind, wind-gust, or stall margin in icing conditions. The ‘performance’ threshold speed is a single value – no spread. The assumptions and procedures associated with use of the increased ‘operational’ speeds have to be heeded, e.g. an aircraft making a ‘standard’ Vref + 10 approach is expected to reduce speed before the threshold (or during the flare) to achieve the ‘performance’ speed at touchdown (approx Vref -7). In some aircraft an icing additive may incur a landing distance penalty if the approach speed is increased above Vref +7 i.e. +2% landing dist for every kt above Vref + 7, refer to your aircraft manual.
Also see JAA NPA 14 Operation on Contaminated Runways – (JAR 25 1591 / AMC 25.1591). I will provide a link to this and the AIC when back at base.
In addition, there is a good overview and many references for landing operations, including speeds in the presentation and speaker notes of
ALAR TEM.