Well, reading your message we have:
- You want to show half star in CC => You need the new custom rating colum
- A custom rating column is a simple type => It cannot be a composite column => No chance of template coming from other fields
Taking into account these two facts, I think the only solution is a plugin, where you can manipulate directly the database to assign the value you need to a simple column or search and bulk edit.
BTW: When I was talking about bulk edit, I wasn't thinking about S&R but directly edit. Let's say:
* #read_count:<=2 and #my_rating: False => A set of registers => Bulk
edit of those records => my_rating: 1
See
this thread.