This use case calculates the total of all remaining estimates (in days) of issues and sub-tasks linked to the current epic.
If you want to exclude sub-tasks have a look at the following use case: Total remaining estimate in an epic (no sub-tasks).
Create a new Calculated Number Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional
Number format pattern
Check the results
Once configured and added to an issue screen (e.g. an epic), the result will look like this: