@ JSwolf-My various file sizes are all source generated as needed, not stretched.
@ jswinden-I originally intended to follow your rationale exactly and had prepared my file accordingly, but when I went to upload, the instructions said I had to provide a single upload file that will replace what is embedded (which it now does) AND that the uploaded file has to be minimum 1000px on the long dimension, 2500px recommended. That's why I came asking for advice.
As I said, my current plan is create and upload a file that meets the minimum 1000px requirement, but the question remains about the ratio. Amazon recommends a ratio of 1.6, which I'm assuming is to make a good looking website image, but that will not result in a full screen embedded image. Given the apparent new need to choose, is your advice to not worry about the embedded image filling the screen and go with the 1.6 ratio, or do you plan to continue to submit covers at the 1.33 ratio (whether 600x800 or larger)?