[Keeper of Dol Guldur]

I'm familiar with "rule of thumb", and I see the connection. But even so, a "rule of thumb" is a LOT less rigid a measurement than using an measuring instrument, like a ruler.

Rule;

1. Governing Power, authority.
2. A prescribed direction for conduct, a regulation.
3. The body of regulations prescribed by a founder.
4. Usual, customary course of action or behavior.
5. A generalized statement that describes what is true in most cases.
6. A standard method for solving a problem.
7. A subordinate regulation governing a particular matter.
8. A straight edged strip, wood, metal, etc, used for measuring lengths.

Anyway, I think Tolkien, who was a tremendous etymologist and a Professor of English, knew all the connotations, interpretations and meanings of the word rule, and I still think it makes sense.