No person has a symmetrical face. That said, The posts above give the answer. A good adjustable shaving mirror helps do the eyeballing. On line these and others are quite inexpensive.
Name: shave mirror.jpg Views: 906 Size: 16.9 KB
If you shave with a spike toed straight razor, watch for the ear lobes! Nicked one this AM with a new Kamisori.
Good luck time will work it out for you.
~Richard