When you make a masking shape, turn off the outline.
The shape you see in the main window is then an accurate representation of the mask.
(I know. Basic. You may well roll your eyes.
But I have been wrestling with an old file for days, hating myself when I come across old masks with thick outlines, used already so I can't redo them, but they are bastards to work with - you end up with a masking area larger than the object. Aaaargh!)
Masking tip
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