NWSRG - when the crew does the calculations, we take into account the runway length (or rahter, the charts we use take it into account) in determining how much weight we can lift. Once we've determined the actual a/c weight at take off we then make the decision on whether to use Flex or Max thrust. Providing you use the charts correctly, the runway length is factored in. (Sorry, am not going to get more technical than that, you'll just have to take my word for it.
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As mutt says, the FADEC doesn't know or care how much runway is in front of you, all it cares about is how much thrust it can give - and that depends on which detent we've set the thrust levers in. (And we've worked out which detent to use from the charts - geddit??!)
Incidentally, yes the aeroplane does measure it's own weight. Also, it does know which runway it's on because we load that info into the box during the preflight, but this is for the navigation part of the computer (so it knows where it started from!), not for the performance bit.
Hope that makes it a bit clearer.
t