However, reducing to idle thrust at high altitude will not get you as low an N1 as you can achieve at lower altitude... I noticed this on my previous A/C as well, as you descend, idle N1 appears to decrease. Anyone know what the technical explanation is (while I go search for my anorak)?
If you're talking N
1 speed, then you're talking fan speed, which isn't really related to the gas generator engine idle speed (N
2 or N
3, depending on your engine).
As you descend and drag increases, the fan speed will vary at idle...but it's not fan speed here that concerns us, but the gas generator speed, which is your power section (N
2 for a two-spool engine).