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 ?