Actually, the thrust is not strictly proportional to N1. Generally, 1% N1 change at TO means about 2 or 3% thrust change. Thus for a 25% TO thrust reduction (max permitted under ATM rules), about 8 - 12% N1 reduction is seen.
Also remember that the first few % is - by far - the most significant effect on engine life. So once you pass a 5% N1 reduction, any further economy is minimal, and in fact the increased fuel burn (lower ROC, longer ops at low altitude) may eat up the engine life savings.
So don't dwell exclusively on engine life, without considering other factors.