Is there an expression for a principle that is a close approximation to the truth, but quite possibly has exceptions? For example, "Do not kill". This is a good first approximation, but almost everyone would make allow except for self defense.
I don't like the word "general", because it is often used in the sense of meaning universal, such as "The general theory of relativity".
I don't like "first approximation" or "heuristic", as they could be unfamiliar to people without a scientific background.
UPDATE: Another word I though of is "preliminary"