[Adoby]

The internal reader in calibre already handle this by embedding the position inside the book file.

I think it would be more reasonable to check if Bookreader has a similar function to remember last position that can be used for this. Either by embedding or by keeping track using a hash of the book or some other way.

Actually I think it would be perfectly reasonable if you asked the maker of Bookmaker to make Bookmaker use the exact same method of keeping track of the position as calibre use. There are other areas where things that calibre add to epub files are used by reader software. Calibre is BIG!