Best Practice to Store an Admin Configured Variable
I have worked with databases that had a settings table, which was table with one row and a lot of columns, where each column represented a value that was configurable, and should hold true everywhere in the application.

As I believe most DBMSes have an upper limit on the number of columns a table can have, there are limits on this, but it can be a practical solution for setting what amount to configurable global constants for the application.

That said, given that there is a logical place to store this information, I would agree that your suggestion (store it with the rest of the manager data, and a special category (default)) makes perfectly good sense, and should make queries a bit easier to write, and to follow later.