Just out of interest, where does the 10.4325" value come from ? The page sizes are generally x.xx" (2 dp) add on a bleed margin 0.125 and you might get 3 dp x.xxx" but 4 ? is strange. something like this https://kdp.amazon.com/en_US/help/topic/...#covercalc
Gimp might not be the best tool for this either, Bitmap graphics ok, but any text on top is best done with a DTP program such as Scribus. The same applies if Amazon are looking for a PDF, use Scribus.
Gimp might not be the best tool for this either, Bitmap graphics ok, but any text on top is best done with a DTP program such as Scribus. The same applies if Amazon are looking for a PDF, use Scribus.