The watched tag in Theater View is actually controlled by the expression: TVInfo(WatchedDisplay)
To check what it is calculating in Standard View, you need to right click on the header -> Add Expression Column and put it in.
The threshold is set at 95%, so no, you don't necessarily have to sit through all the credits. There are other ways that a show may not appear as watched though. There are some examples in this thread:
http://yabb.jriver.com/interact/index.php?topic=84964.0The part I am not exactly clear on, and maybe Matt or MrC could shed more light, is how the Bookmark tag works. From what I've read, the Watched function works off of Bookmark vs Duration, but it does not give a simple Yes/No answer. It can also provide Date. So is it looking at the Number of Plays or Last Played tags at all or is all the information carried in the Bookmark number?