Ah, I misinterpreted that as sarcasm. :)

With respect to Becco's method, you would have been better off using the knife tool with plane option.  This guarantees the planarity of the edge for symmetry and HN.  Though you may need to fix the object location to assure that it is against one of the world system coordinate planes since the Symmetry object works along them (this also can be changed by adjusting the Symmetry object's Coords before childing the Polygon object).

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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