This may seem like a strange question but when I am running pen and paper campaigns and the party enters a new area from somewhere high up (like a mountain overlooking a city) I am always unsure about how to describe what they see because I have no sense of how much distance one can see given the curvature of the world and a given height. Knowing this information would help me design my maps as well as give an appropriate description to the players.
So, leaving out things such as atmospheric conditions and other high objects in the way (assume the vantage point looking out is by far the highest object), how could I calculate the field of view and distance for an average human? Other simplifications may be necessary; I'm just looking for a quick and dirty estimate (an equation) that is realistic enough for an RPG where everything is in the player's heads. However, I have seen very complicated answers on this SE so if you want to add more detail please feel free. For the purpose of this answer, assume that we are dealing with Earth.