Mutt,
The A320 (which I believe this was) is certified for an engine failure at the critical point (v1) at max flex, which by definition is climb power, which by definition (on the 320) is 75% TOGA, without increasing thrust on the good engine. Therefore all single engine performance at max derate (which BOS 27 probably is) is calculated at 75% TOGA.
Can't be arsed digging the reference out, do it yourself and learn something
LD