I find that the bit rate reported by the web UI or SmartPSS spends most of its time near the max bit rate anyway so maybe I should just use CBR @ 4096 which is the max in the dropdown.
VBR is lighter on the storage if you can convince yourself to not set the quality to max.
Usually I recommend running cameras at the highest possible bit rate for your chosen FPS, because when you capture something important you will want every bit of detail you can get. Only cut back on bit rates if you can't afford enough storage to have the retention time you want.
It looks like I'm now getting about 2800 kbps with the settings in my first post and the max bitrate set to 4096. That's during daylight with a mostly static scene of short grass, street, and driveway. The image quality is better than when I had the max set to 2048 but not quite as good at CBR @ 4096. I only have two cameras so retention time isn't a huge issue right now but we'll see how it goes as a build out.