As many have said, this is not an exact science. Thus there needs to be some awareness of the situation and a philosophy, perhaps, of which is better; a go or stop takeoff. As was seen last winter, trying to stop well down a slippery rwy might not always be the best idea. This is where it is necessary to understand that the figures are for guidance and there is no guarantee.
Having just read the latest publication from UK CAA about operation from contaminated ryws it is apparent that braking action info is also very subjective and for guidance only. It is emphasised that stopping distance is not guaranteed, neither is the directional control in crosswinds v braking action. Judgement is required. I wonder how much buffer airlines build in to their figure, and how much training is given to appreciate these parameters.