Denti - our company does not give us the option to see or select from a list of FLEX Temps. Its MAX FLEX or TOGA. This is done purely for financial reasons. They are in the habit of phoning crews who used TOGA and asking them for an explanation. Its intimidation, and it works on many, hence the 'trickster' attempts at getting around the numbers (hello callmeJB).
Your story about the overrun is instructive.
Yes there are lots of margins added to our Performance calculations, but always remember that the base data is derived from test aircraft, perfectly maintained by the manufacturer, with new brakes, on a standardised runway surface, with a test pilot at the controls who is pre-warned and prepared for the RTO.
Ordinary sods don't have all those advantages.
Step outside the ideal world of razor sharp decision making and reactions and your 'additional margins' start eroding.