I would like to add a field to my library called 'Sort Name'. I would like the value to default to the Name of the file...unless I change it.
You can do this for sure. You can use a system similar to how [Album Artist] and [Album Artist (auto)] work. But you don't need to for this particular task. I'll explain another way below.
Then, in Theater view, I can sort by 'Sort Name', and get all my grouped movies to be together.
An example would be The Dark Knight Trilogy...
Batman Begins
The Dark Knight
The Dark Knight Rises
I do this in my Standard View and it should be similar in Theater View. My idea works by using the [Series] and [Episode] tags to identify movie series, and their ordering. So Batman Begins would be [Series]=Batman and [Episode]=1 . The Dark Knight: [Series]=Batman , [Episode]=2 .
Now you need a custom field to help the sort work properly. In the Mange Library Fields dialog, create a new field. I called mine [Movie Sort], but you can use any name you want. Make it a Calculated Data field using this expression:
if(isempty([Series]),[Name],[Series] [Episode])
So it will sort by Series and Episode if they exist. If not, it uses the Name field.
I think that will do what you are after, and make it easy for you to maintain.
If you still want to know how to do the "put in one value unless I change it" thing, I can outline that too.
Brian.