Snowonice,
There have been a few threads in the Tech Forum on this problem.
The 15 per cent fudge factor traditionally has addressed the simple wet (not flooded) runway case and, for that matter, operations on gravel using seal data.
For flooded or ice/snow/slush contamination it all gets a whole lot more complex and the approaches adopted in trying to allow for the imprecision of rolling and braking data in the real world scenarios lead to an interesting variety of engineering guesswork.