The lens' focal length is independent of its position between film and subject.
Extension can be thought as starting from two different positions: the film plane and the lens' infinity position.
The distance from the film plane to the lens' rear node when the lens is focused at infinity is the lens' focal length.
The distance from the film plane to the lens' rear node when magnification (m) > 0 (m = 0 means the subject is really, really far away) is f * (1 + m). By subtraction, extension from the infinity position is f * m. f is the lens focal length.
For lenses commonly used on LF cameras (not retrofocus, not telephoto) both nodes (front and rear) are close to the diaphragm.