Haroon, there is no specific factor recommended for in-flight assessment. The issue revolves around safety, which as discussed in previous threads places the responsibility with the Commander.
Thus, the judgement of safety (acceptable risk) could involve arguing why a 1.92 factor was deemed safe before takeoff, but not necessary before landing. And as you indicate, it’s better to consider that in-flight rather than off-the-end of a runway.
Use of in-flight data depends on a good understanding of the basis of that data and the assumptions in it. Many manufacturers are revising their ops-manual data; see the threads relating to OLD.
http://www.pprune.org/tech-log/50302...-distance.html
http://www.pprune.org/tech-log/49803...ns-airbus.html
Even with improved data, a Commander will still have to consider additional factors; a good guide to these is in
AC 91-71 Runway Overrun Prevention.
BOAC, IIRC the (UK/EU) 15% only relates to contaminated runway operations. There is mounting evidence that this factor is insufficient in the wide range of conditions / reports for this case; and that the landing data may also depend on the presumption that contaminated runway should not be used routinely (avoided) or additional mitigation taken.