I've heard James Fadiman reporting twice that it is 10% of a recreational (1-3g) or therapeutic dose (3.0-3.5g), but certainly not 10% of a heroic dose. This study went with 0.5g which is 10% of what's considered a heroic dose.
The effective dose is considered to be 6mg if psilocybin or 0.6g of dried magic mushroom, thus the amount given in the study is very close to being detectable (and incidently was in most of the cases) which I don't think you can charaterize as a microdose. James Fadiman expressly stated a microdose would be a dose that you couldn't detect and would forget you had taken while going on about your day.
If one is taking 0.2-0.3g of dried magic mushroom this would be, without question, not getting high.
During my youth, I had strong trips with .5g and meh one's with a few grams, sure it's not the norm but it's not an exceptional occurrence either.
My experience told me that the stem of the smallest mushrooms are a lot stronger than those of the bigger one but that an heuristic not an hard rule.
If I wanted to microdose psilocin (for which psylocibin is a prodrug), I would order 4-AcO-DMT¹ fumarate (another prodrug for psilocin) from a reputable Canadian chemical supplier. It's almost identical to mushrooms, the difference being that it is predictable and somewhat easier on the digestive system.
1) https://en.wikipedia.org/wiki/O-Acetylpsilocin https://psychonautwiki.org/wiki/4-AcO-DMT
I know some English names can go both ways like Evelyn or Shannon etc, but is Emily one of them?