WRT the picture from
Pelican's Perch #18: Mixture Magic shown above, a bit further below the same article states that
This second and much more serious red area holds true through the entire spectrum of power settings, so it is much more serious. Virtually all factory big bore engines suffer from this uneven power distribution, which sets an artificial limit on just how lean we can run. Uneven mixture distribution causes the entire lean-of-peak-EGT region to become a red zone, not available for use!
So actually at least the poor armchairflyer is solidly confused concerning the proper mixture setting (except for taxiing, where
AFAIK any degree of leaning does no harm in that almost idle regime).