if I watched more than 90%, then I probably finished it.
Less than 10% - insufficient to decide, likely need to try once more
Somewhere in between - I probably dropped off in the middle of movie - - > DELETE
Except for the delete reaction, that is pretty much what the Watched() function does. I'm almost positive that it uses 10% and 90% completion as you do. The somewhere in between would mean go back and finish watching for most people. Perhaps if you don't like a movie, watch it for more than 10% and decide not to finish it, then set the [Rating] field to one star and exclude those movies from the View, and then delete them later.
EDIT: Yes, you can tag files in Theatre View, though setting the [Rating] field isn't pretty. There is also an [Access Rating] field that you could use. I'm not sure what that field is actually intended for. It could have been used for "
Access Control" which has been replaced by "
Users" functionality, so could be available to use... or not.
MC isn't going to delete files for you based on an Expression, but a View or Smartlist could be used to show movies that should be deleted, after you have partially watched and rated them.
This is the latest change log that modified the watched() function. The thread is interesting, but most of the discussion happened after the change was implemented.
18.0.126 (1/30/2013)
3. Changed: Tuned how the Watched(...) expression and watch marks in Theater View work (
http://yabb.jriver.com/interact/index.php?topic=77602 ).
Although there was this minor tweak.
23.0.29 (8/1/2017)
6. Changed: The expression Watched() will show something like "10 of 12 watched" for groups of files.
There were a couple of TV specific tweaks as well, that shouldn't affect you. I'll let you search for those.
Read all the threads people have posted, and you will learn how the issue can be and has been addressed. Then you will be able to implement a solution the way you want.