First - a matter of definitions:
FLEX is based on A RATING plus an assumed temp. The RATING can be either the nameplate rating of the engines, or a derated value (can be either -10%, -20%, etc. OR an earlier engine rating). Thus under a 20% derate (=80% of nameplate), then FLEX might take you to as little as 60% of nameplate thrust (=80% x 75%).
If the EFATO after V1, you can advance to the baseline rating - in other words VMCA and VMGC have not been compromised by the FLEX selection.
However, if the rating used is -10%, for example, and a FLEX TO is based on that -10% value, then you can only safely advance to the -10% rating. If you go to full nameplate rating then all bets are off.
