It seems to me that it’s quite tricky to get text above and below the staff to appear at ‘seemingly’ the same distance away. This does vary from font to font - so I presume the ‘top line’ metrics are relevant, but even Academico is not immune.
It would be nice to have a separate vertical distance setting for text below the staff.
The distance is measured in each case from the base line of the font (an imaginary line along the bottoms of letters without descenders). In the case of text items below the staff, the height of the letters fills in the gap. I have wished for text below the staff to be measured from the tops of the lowercase letters instead of from the base line, but this seems not to be the done thing; at least, Sibelius, if I remember, behaves the same way as Dorico.
I don’t think it is the baseline, because if you make the font size much bigger than the set distance, e.g. 32pt and 0 spaces, the Below text still clears the staff.
Certainly, some combinations of Vertical distance and Protruding distance seem to work better than others.
Isn’t that collision avoidance?
Not necessarily. Staff lines are not considered for collision avoidance.