The numbers in the book allow for crossing the fence at 50ft, flareing to touchdown, and stopping using full brakes without reverse thrust. The sum of of the above is multiplied by 1.67 to allow for variations in technique, touchdown point, runway friction and all the other uncertainties of life. Attempt to land in less than that and you are out of warranty, insurance cover and probably your job. The unfactored distance will have been demonstrated by a test pilot, but he will have been trying hard and flying the landing exactly according to the book, getting the brakes on sharpish etc. There has been a lot of argument about the 1.67 factor, but it can be substantiated - sort of - in terms of the sort of performance people generally put in. So it is worth respecting.