Perfectly possible to recalculate TO perf at the hold - but is it a good idea?
I see plenty of mistakes done in the figures at the gate with 3 people on the flight deckand no interruptions, and get very nervous when we need to redo it during taxi or when ATC are trying to talk to us at the same time.
I guess its just one of those areas where our needs diverge - ATC want max runway useage, all I want is my a/c operated as safely as possible. Mind you, why anyone would want to recalculate when they already have figures for a shorter take off run is beyond me. When I was on short haul I just did the worst possible case figures and took out the shift if we were full length. If you want full length, asking for it as early as possible seems to work....