Getting a model to hit same exact mark
One option, if you don't mind the actor keeping her head still, is to be partially lying down. She could be in a recliner, leaning back, with her head against the headrest so she never moves it.  She would feel the headrest behind her so she could tell if she'd turned her head, and it wouldn't "drift" left or right because the friction of the headrest would keep it in place.

Of course you would want to shoot all her lines in as short a session as possible so she didn't get uncomfortable - like 5 minutes for all the lines and retakes.