I find some of these incidents strange, speaking as an old pre glass/ fully automated pilot.
2+2 always equals 4 the same as cruise power for the applicable flt level always equals the applicable cruise speed.
If the ASI says otherwise then the ASI is lying. The only way I would be looking at idle thrust at high altitude is if I were wanting to descend.
All that is needed to fly at FL380 in fact is an altimeter, a thrust indicator and a manual of power settings.
Emeritus