My colleague, Michael Sampson, recently posted his equation for productivity.
In his essay, he describes a productivity framework for a knowledge or information worker
1. Having a clear picture of what I want to achieve (clarity).
2. Removing irritations and interruptions that break flow (interruptions).
3. Having a brain that is rested, energized and active (rested).
As an equation then: C - I + R = P
His entire essay is available on-line, here. A worthy read!
UPDATE: GTD FORUM LINK EDITOR BROKEN. HERE'S THE URL: