About Airbus it says on the Performance section of the FCOM THRUST OPTIONS - FLEXIBLE TAKEOFF - REQUIREMENTS:
"-Thrust must not be reduced by more than 25% of the full derated takeoff thrust.
- The flexible takeoff N1 cannot be lower than the Max Climb N1 at the same flight condition.
-The flexible takeoff thrust cannot be lower than the Max Continuous thrust used for the final takeoff flight path computation (at ISA +40)"
(you can replace N1 for EPR in other engines)
A bit late but hope this helps.