Col, I go on the exit pupils.
In my rough rule-of-thumb:
Low magnification is using an Eye Piece that gives an Exit Pupil of 3mm or larger.
Medium magnification is using an EP that gives an Exit Pupil from 1.5 to 2.6mm
High magnification is when the Exit Pupil 1.4mm or less.
Although, my rule may break down somewhat when using large Dobs (but I do not have one on those).
You may have noticed that I missed from 2.7mm to 2.9mm. I classs this as a "flat" zone, that is not really optimal for viewing any class of objects.
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