Sequencer - How do you setup the camera in filmback settings so you can render out specific pixel count

In 4.16 or 4.17 I need to render out a very specific pixel count but there is no pixel settings in the camera. How do you translate the filmback settings to a specific pixel count ???

You cannot just render out any pixel count in the Render movie panel / capture settings because if the filmback is set up to one thing and you try and render out some random pixel count it adds black letterboxs in the render ???

How can you frame a shot if you cannot tell the exact pixel count ?? Unless your stuck using presets - ??