Found this very thorough write up on the Clone Stamp and thought it might be helpful with images like the one Boys.
Detailed write up ... best info about half way down on Cloning Source Offset Adjustments.
https://www.smashingmagazine.com/2010/03/the-ultimate-guide-to-cloning-in-photoshop/
Great video explaining how to mirror: https://www.youtube.com/watch?v=rrNgJZkUd9M