Exactly why the planar knife is so useful.  If you aren't exact in your cut so as the plane is 0 on an axis, you can simply check any one of the resulting points for the X,Y,orZ value and translate the model by that much to guarantee that all of the points are 0 on the axis before applying symmetry.

But, as Anders suggests, select all of the points (in Point mode) that are supposed to be 0 on the axis and Structure->Set Point Value to 0 on that axis.

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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