Found that this guy is using Empty Plain Axes as the Object for Mirror modifier, which allow him to define the rotation of the mirrored instances… which is fantastic, still he is using it at regular 45°/90° angles, things get tricky with random angles like the pic below…
I am sure this is just a semantical/mathematical thing, but clearly, just setting up/rotating the Empty to fit that one weird face does not produce any results…
1.Select the face
2.Set cursor to selected
3.Shift+Num1/3 or 9 to align your view with the face normal
4.Create an empty and then select the align to view checkbox
5.Set the empty as the mirror object
Now the next question would be, how would I rotate/align that face [well, the whole object, right?] to a Z or whatever axis?
You know, so its nicely aligned, with 0,0,0 rotation …
Parent the object to that empty
2.Select the empty and go to the object panel, Set it’s X, Y and Z rotation to 0
3.Press Shift+S and select “Cursor to center”
4.With the empty still selected, press Shift+S and select "Selection to Cursor
5.Select the Object and then Press Alt+P and select “Clear and keep Transformation”
6.Press Ctrl+A with the object still selected and select “Rotation and Scale”
7.Feel free to delete the empty since the the mirror can now be used normally.