On the FMGS status page you'll have an idle factor - that affects the idle thrust set in a managed descent - your company will have decided it.
On top of that, FCOM DSC-70-40-40 P 2/4:
The FADEC has the following three idle modes :
Modulated idle ‐
Is regulated according to :
• bleed system demand ‐ Is selected :
• In flight, when the flaps are retracted (FLAPS lever at zero position),
• On ground, provided reverse is not selected.
Approach idle :
‐ Is regulated according to aircraft altitude, regardless of bleed system demand.
‐ Is selected in flight, when the flaps are extended (FLAPS lever not at zero position)
‐ Allows the engine to accelerate rapidly from idle to go-around thrust
Reverse idle :
‐ Is selected on ground, when the thrust lever is in REV IDLE position.
‐ Is slightly higher than forward idle thrust.
It gives you a buffer for head/tail winds, big temp changes, half asleep, distracted and bored pilots and short cuts.
Consider re-inputting the cost index on the FMGC PERF page every time you pass a constraint to allow the system to recalculate the profile and optimise your descent. Open Descent is rarely going to be more fuel efficient.