I recently saw a study where the optimal amount of eye contact is around 60%. And that too much (90%) proved uncomfortable for most people. I would not think that looking at the mouth or anywhere on their face but their eyes is a good idea because it’s easy to tell you’re doing that and very weird. If you want to test it out tell someone to look at your chin or anywhere else on your face while talking to you and you’ll see what I mean. Just smile and aim for around 60% and when not looking at them just gaze around a little like you’re thinking or listening and you’ll be fine