Most faces are not fully symmetrical. In traditional Hollywood filmmaking, actors, especially female actors, always had or believed they had a “better side.” In your situation, there are no formulas — there never are. You will want to look at the face and decide which side you want to key from, as well as the look. Hard light or soft? How much fill? How much back light? How bright the background as well as its colors and what objects/walls/lamps appear back there, etc. The fun part is, figuring all this out (often in a very short period of time) is a constant challenge and learning experience.
Rick Wise
Cinematographer
MFA/BFA Lighting and Camera Instructor Academy of Art University
San Francisco Bay Area
https://www.RickWiseDP.com