[franc]

Thanks!
Oh, indeed, your plugin is a big one

I had a look into Moon Reader's sync file (on Google Drive / DropBox), called name_of_book.epub.po and this is very a simple file, contains just that, e.g.:

1469998681113*14@0#2952:23.0%

and after reading a page it was:

1469998681113*14@0#9043:23.5%

or:

1469998681113*15@0#26438:27.5%

Only to change the percentage at the end in Google Drive doesnt make Moon+ to jump there.

I "only" need an interpreter of Calibres positioning and this Moonreader positioning file.

EDIT: I even don't know how Calibre puts its reading position
Kovid said:

...calibre stores them inside the epub file...

So that is a start. So within Calbire I edit the epub and I found under:
META-INF/calibre_bookmarks.txt
the content:

calibre_current_page_bookmark*|!|?|*17*|!|?|*/2/4/234/1:242

which I strongly aspect as the right information

I asked the developer of Moon+ Reader how these .po-files work. Hopefully he tells.
If he tells maybe I am able to translate those two formats to each other and update the po file in Google Drive with a plugin. Then I guess I would use the Kobo plugin as a base to develop, but maybe I am widely overestimating myself and underestimating that task :O

EDIT 2018-05-15: The Dev. never told me his secrets, didn't want to
I thought I discovered the scheme, the long numbers were just time-tags (unix-style) and tried to emulate some own reading positions, but this always failed, never I got that working, so I concluded that there is some secret thing in Moonreader to check if the position is really put by Moonreader.
I gave up then