ffmpeg – Re-Calculate Average Framerate of MP4 Video Upon Decoding

For an example mp4 video, I am seeing differences between the header metadata values for at least some fields (e.g. nb_frames) and the actual values retrieved upon decoding the video (i.e. nb_read_frames).

I have found documentation for how to decode and get the correct frame counts and duration, but I am trying to do the same with cross-checking avg_frame_rate. Is there a way to decode and re-calculate the average framerate for an mp4 video?

I am able to run a basic analysis (is this correct?) via:

ffmpeg -i input_file.mp4 -f null -

But I don’t understand how to interpret the results, or if I can derive the average framerate from these values. Can I just divide the frame count by the duration given in the output of this command? Or is it more subtle?

Any help here is appreciated. Thanks!