Willie, "…
you must at least know what the margin of distance is in the event that you miss the TDZ."
You appear to misunderstand or incorrectly define TDZ.
The 'TDZ' (fixed distance runway markings) is not related to actual runway length, thus there is no relationship with the aircraft landing performance, which is based on the LDA and local conditions (LDR).
The TDZ should not be confused with, or replace the expected touchdown position.
The distance safety margin is applied to counter many unknowns or variables in the computation; it is not for the crew to 'use'.
Because there is no relationship between TDZ and the stop end of the runway, it is not possible to be assured that the remaining runway distance will be sufficient to stop the aircraft.
FD
