I noticed that Plex got the credits timing wrong on a movie and the “skip credits” button came up way too early. I searched for ways to set this manually, but could not find any.
I saw other posts asking for this, with either no response, or a link to a help article that just describes the skip credits feature, but doesn’t say how to do what the OP asked.
The closest thing I can see to a solution is to view the XML for the movie and see where the startTimeOffset tag says the credits start, but I don’t know how to edit that file to manually set the time for the credits starting.