I suggest to use constant quality encoding instead of a constant bitrate, this way encoder will automatically adapt bitrate for a particular scene. This is a much better approach, it will give you better quality and a smaller file at the same time.

For example, encoder might choose 1Mbps bitrate for a scene which is mostly static and 20Mbps for a part of the video with a lot of movement. Your 8Mbps constant bitrate will be an overkill for the first scene and too low for the second. Let encoder decide the optimal bitrate.