There are underlying mathematical influences that determie how a film gets put together, which are amazingly consistent, seemingly independent of the films themselves. Over the years, I've come to rely on these influences - navigation points - as I work on each film. For instance:

2.5 - an audience can only process two and a half thematic elements at any moment;
14 - a sustained action scene averages out to fourteen new camera positions a minute;
30 - an assembly should be no more than thirty percent over the ideal running length of the film.
- Walter Murch, on the art of editing film