OMG, isn’t Amazon’s Cover-flat template utter crap! You plug in your trim size and your number of pages so it will calculate the size of the spine, and it then lets you download a template which will not be the right size.
What’s the point of that then? Grr!
I’m currently getting around it by uploading a blank file for the cover flat first. Then I get an error message that says “Your image should be [x by y dimensions]!” And then I go away and make the file to those dimensions, randomly guessing at where the gutters and spine should be.