I'm not sure why you are so determined not to believe me so I have done a screencap from my very own PocketBook 360 displaying 14 books - Book 5 to Book 18 - of a test series sent from Calibre using the template I originally proposed.
Unless my eyes deceive me, they are all in the correct order are they not?
Edit: Then as a further test I renumbered Book 5 as Book 1 and resent to the PB360. It was still sorted correctly, at the top of the list, before Book 6.