That equation can be found in this odd article here: Evolution: biologist George Price’s life and death – Vox.
It tells the story of George Price and how his extreme altruism led to his death. Well worth reading, especially if you think self care is bunk.
In short, take care of yourself to some degree, or you end up not benefitting anyone. And if you are not benefiting anyone, you are not being altruistic after all.