In a quite interesting document called H.264 Primer, a simple formula is given as an hint to compute the `ideal’ output file bitrate, based on the video’s characteristics:
[image width] x [image height] x [framerate] x [motion rank] x 0.07 = [desired bitrate]
where the image width and height is expressed in pixels, and the motion rank is an integer between 1 and 4, 1 being low motion, 2 being medium motion, and 4 being high motion (motion being the amount of image data that is changing between frames, see the linked document for more information).
So for instance, if we take a 1280×720 video at 24 FPS, with medium motion (movie with slow camera movements, not many scene changes…), the expected ideal bitrate would be:
1280 x 720 x 24 x 2 x 0.07 = 3,096,576 bps => approximatively 3000 kbps
This is purely a hint, and in my opinion, the only way to accurately find the ideal bitrate is trial by error 🙂