Mirroring a model

i have this case of facia asymmertry , my purpose is to evaluatr zygoma position
steps i followed
1 segment the skull
2 segment zygoma
3 trnasform zygoma to model
4 mirrror zygoma around x axis in suface model/ advance/ mirror

however i got it displace and not mirrored perfectly arounf x axis , i tried transformation to 3d position before mirroring and didnot fix it

any idea how to fix that ?

In general, x axis of the coordinate system that a mesh is defined in is not aligned with the anatomical left-right axis and the origin is not in the center, therefore after mirroring you need to register the mirrored parts to the original surface, using one of the many registration methods available in Slicer.