PJ2- I almost agree with you:
the charts for actual landing distance are produced as you discribe - the only problem is: they are produced by test pilots, who have several tries to achieve the best possible performance. I don't know about you, but I am no test pilot. I am what you call in Germany a "line-swine". And I have pax in the back who do not appreciate being slammed onto a RWY at 600 fpm. All this influences my habbits of landing a plane, and I do land harder in the winter on a 2000m RWY than in summer on a 4000m. But I am a victim of my habbits. Do you get what I mean? I always try to consider the weaknesses I have - not being a test pilot, being tired, fatigued, no new brakes, tyres... and add my 30% to what the chart gives.
Nic