Looking at the MP4 encoding options, I can set an average bitrate of 768-14000 Kbps when the frame size is 720 x 480. That's fine because I want to use a bitrate of about 1500-2000.
Some simple math says that there is 2.7 times as many pixels at the higher resolution than the lower. Using 2.7 * 1500 (desired rate at lower resolution) says I should be looking for a bit rate of ~4000 at the higher resolution.
What I find curious is that, when I change the frame size to 1280 x 720, I see the bitrate range limited to 6000-30000. Why is PD limiting the min rate to 6000? Is there a way for me to coax PD into being a little more flexible with that number?