Transition Level in A320
In the Systems Related Procedures of the FCOMs of A320 it is now mentioned that the crew should NOT modify the default TRANS FL value in the PERF APP Page, if it slightly differs from the Transition Level specified on the approach chart or reported by the ATC.
Can some clarify why we SHOULD NOT modify the TRANS FL value. And what does 'slightly' mean?
If the Transition Altitude of an airfield is 4000ft, and if the default TRANS FL on the PERF page is 4500 and that reported by the ATC is FL55 isn't it better to modify the TRANS FL on the PERF APP page to FL55 so that the pilots will have enough time to reset the QNH and commence the Approach Check List.