Gaunty: There is nothing particularly magical in the 1.67 or 67% fudge factor you mention. It is just a nice round 40% buffer calculated from a different viewpoint.
Actual landing distance x 1.67 = required runway length is the same as saying that 60% of the available runway length should be the distance you plan to use. It probably goes back to the same person who decreed that performance calculations only count 50% of the headwind and 150% of a tailwind. They could just have easily been 49% or 151% but round numbers must look neater to the boffins.
I guess that if a pilot had set the rules it would have been a 1.7 or an even better 2.0 factor calculated your way.
Capt. Fathom:
Also, the autopilot stops using the Glideslope at around 50' and starts to flare based on certain parameters. A lot can happen between 50' and touchdown.
Like about 700 meters of overshoot ??
You will have to pull the other one, mate.
Edited to be cheeky to Capt. Fathom