The amount of extra runway required will all be a bit academic really, if you were field limited. You will get away with it if obstacle limited and an engine doesn't fail.
The whole background to Assumed / Flex temperature thrust reductions is to take a 'lighter than limited' aircraft to a LIMITING situation by utilising lesser thrust as though the aircraft was Field (or Obstacle) limited. Thus, you have artificially put yourself in a limiting situation, and there is no additional field available. In an RTO situation from V1, you're screwed!
For an all engines departure (no failure), you'll be utilising the 15% extra margin provided for Field requirements, and you'll PROBABLY get away with it, but if an engine fails.......refer to the RTO comment.
Regards,
Old Smokey