Greetings,
I am on the A330, however, Single Aisle or long range dont matter since they are both common in fact you can plug a thales on any Arbus, except 300 and 380, because they are developped from the same specification 555 from aerospacial.
Having said that, I reinstate what I did before, as per the specification the FMS will keep either as close as possible or be on the optimum profile, this is the conservative side.
Now for MAN FLEX 37.5:
The Idle you have noticed is what is refered to IDLE + Delta, it is there to help manage the descent and stay in the brakets, and not for the Anti-Ice, it allows the FMS to command further thrust reduction when its profile reaches the bottom of the braket during descent, as the last mean of loosing energy.
Optimum profile is flown at optimum speed, so any time you depart from that scenario you do better (or worst than the FMS)
Now I recommand you to have a chat with Enginering department and check is they have a value for your aircraft IDLE FACTOR, if they do Then use it if they dont ask then to survey Engine PERF, I am sure they wll come with something.
Happy Landing
