Hello! Thank you very much! I'm pretty sure that overscan size in precentage is very un-intuitive in most cases: it's major goal is to render images for compositing with undistorted footage or to compensate 2d trackng. When one performs such tasks in Nuke or Natron, needed values will be expressed in pixels, not in precentage. The order of elements could be reconcidered, thanks!
Nice addon! I would just suggest adding a feature that allows the user to set the overscan as a percentage, because that's how I usually calculate my overscan, not in pixels. Or it would be really nice if you could enable the overscan and then set the overscan percent using the output resolution percentage that's already listed in Blender and have the camera's focal length update dynamically. I would also recommend changing the order of the "Overscan Size" setting to right above the "Enable/Disable Overscan" button. This to me would be a little more intuitive... First, I choose my camera, then I choose the overscan size, then I enable it. Thanks for your work on this!