I use fundies of light as the baseline to put down a light source somewhere I know it is a cliche at this point but it all goes back to the basics from Sycras light fundamental tutorial series.https://www.youtube.com/watch?v=0q-LZVFZuGE
That is the basic logic, after that, just personal taste and laziness. When dealing with challenging things or I just don't want to complicate things I prefer to have the light source fixed on one side of the face. That makes it easier to find the geography of the face which is my eternal nightmare.
It can also serve kind of a rim light too but that is not realistic and I know it is not realistic but it makes things easier.
Shitty attempt to show it man working with a brush like this is PAINFUL!