I much prefer the OP's 2/3 ratio for the ebook cover, which to my eye is what the Big Five and most other commercial publishers use. It looks better on the Amazon store, which is where sales are made. (When Amazon has its way, the reader never sees the cover on his/her/its Kindle or app, because the book opens at page one, chapter one.)
I think the OP would have a happier experience by using Sigil rather than Calibre, and uploading a validated epub.
Happy 2020! -- NJ
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