You'll need a way in your database to identify which records are "owned" by a given manager and linked either directly or via a table to their account name they use when logging in. I'd assign a "ManagerID" value to a field and use a table where ManagerID is a serial number and their account name is stored in a text field. I'd have a script that uses the manager's account name (Get ( accountName ) ) to look up their manager ID and puts it in a field or variable with global storage. (This way, if you change a manager's account name, you only make a change in one record.)
You can then use this global field or variable in the "lock expression" that controls record level access.
You may want to:
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a detailed description of how to set this up.