It is more of what your eye perceives. Some people can see differences that others don't or maybe a monitor limitation.
Personally when I am trying to figure out the bit rate I will start at a number, so in this case I would do 10,000, 12,000 and 14,000 and take pics at each one and then flip thru the pics and see if I notice a difference.
If not then I no bitrate is too high and then try the next set of numbers.
The higher the bit rate the more storage that is needed, so you want to run as low as you can before you see degradation.
Once I am down to a few bitrates then I will do a digital zoom and see if I notice a difference. A higher bitrate can usually allow for some digital zoom.
But again it comes down to your field of view and eyes.