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After you show the window, the left-top corner of the label is at ui.label.mapTo(ui, QtCore.QPointF(0, 0)). However, the very placement of the labels makes me look into ui.layout.contentsMargins(). One way or another, I get (9, 9). For you, the layout might differ, though.
Beware that it takes a while before the OS places the window wherever it considers the best, so don't get the coordinates until the window is painted (or just wait a second).