Hi jets are far more critical to things like temp and weight to determine their takeoff landing distances! Hence why going off the figures in the manual is vital and not a manufacturers marketing document
Even in pistons how do you get your high altitude per cent power range figures from if the manufacturers speed are way off?
If your aircraft has almost time expired engines, is bug ridden or covered in dents add a percentage to cover or know your aircraft
Takeoff in a jet!! all you can do is stand on The brakes hit max N1 release the brakes and wait for VR!
If your heavy or it's hot that can take forever and the takeoff distance can vary considerably from light and cold!
There are so many other considerations is the runway wet ? How much headwind etc the manual is vital!
I believe a Citation operated out of Elstree but IMO that was crazy and it did crash! Not heard of the Denham overrun
Pace