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A front-facing portrait is especially challenging, even with a reference.
No one sits at a perfectly square angle to the camera. No face is perfectly symmetrical either. A slight tilt or rotation is enough to break the illusion.
Lighting makes it worse. It can push the perceived symmetry even when the actual structure is not off.
Do all these factors affect the final result?
Yes.
So you just do your best.
In the end, it is in the eyes of the beholder.
The audience will vote, but never unanimously.
Another note: if your hand misreads the tilt of the head, the subject can end up looking much younger or older than intended. One wrong bit of foreshortening can change the whole face.