Excellent explanation, Paul
Twice stupid from PTC's part
First, for slicing the circle in two for no reason
Second (forgivable, actually), for not understanding that when I select that edge, I'm trying to refer to the height it stands on, not the edge itself.
(Or not understanding that the other edge is just a perfect continuation/segment of the previous)
Thank you all guys
Rohit, Martin and Paul
Your tips of auxiliary data resolve the problem with success and elegance.