The Exposure Formula is associated with the quote "Chance favors the prepared mind," so I take it to mean that knowing the formula and using it can help in a pinch when a meter fails or is unavailable.
Ansel's formula requires a reading. No reading, no answer.
This isn't a rule of thumb thing like sunny 16, or an exposure table.
Mark Barendt, Ignacio, CO
"We do not see things the way they are. We see things the way we are." Ana´s Nin