@Ephemerality,
Quality Check is much, much faster for inserting the right exth 113 values.
Don't use "both" -- simply convert to AZW3.
"both" gets you nothing except an ebook that is backwards-compatible with the Kindle DX/2/Original (they don't understand AZW3).
And it comes at a cost --calibre doesn't know how to update the metadata in a dual-MOBI.