dj1, there have been several discussions on this subject for commercial operations in tech log.
The central issue is the justifiable level of safety.
The pre takeoff factored landing distance could be discarded in flight if there is an alternative means of calculating landing distance, and one which provides a similar level of safety as the pre takeoff calculation. This probably means adding a factor according to the circumstances, but a factor on to what baseline distance.
Many commercial operations did not use the standard factors and substituted their own methods in flight. Unfortunately they often used manufacturers’ actual distances as the baseline which differed considerably (no factor or margin depending on assumptions) from the certificated performance in AFM, which is the basis for the pre take of factors.
Recently commercial manufactures have published much more realistic ‘actual’ operational landing data, often with a minimum factor of 1.15, but even with this the responsibility for the safety of the operation resides with the operator who is expected to add further margin as judged before landing.
For public transport ops it might be easier to use the pre takeoff factor in flight as the basis of your justification, which should additionally consider any in-flight changes and the actual conditions before landing.