It's not that simple as it depends on the nature of the source material.
Bitrate varies constantly throughout a film and the encoding software tries to allocate it to where it's most needed.
Fast action scenes or scenes with lots of details require a higher bitrate than fairly static bland scenes and the encoding software tries to redistribute what's available to optimise overall quality.
Generally speaking though the higher the average bitrate the better the quality.
Whether that quality is acceptable or not depends on a number of complex interacting factors and there's no definitive rule of thumb that will guarantee you a certain degree of quality at a particular average bitrate.
Your best bet is to try different settings and see what you personally are happy with.
Even then, all films are different, and you'll find that what works on one won't necessarily work on others.