While not an exact metric bits per pixel can be a useful guide in determining the optimal bitrate for a given resolution.
Calculated as: width x height x fps / bitrate.
Now for h264 and similar codecs the result needs to be interpreted based on the content. If your content is quite static you could likely get away with values around 0.1 though if you want reasonable quality with a lot of movement and colour around 0.2 will produce reasonable results. To get quality closer to the source for high action sports e.g. Australian rules football I have used rates in the 0.3 to 0.4 range.