Sum up the remaining estimates (in days) of issues and subtasks in an epic.
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
Add the following expression:
sum(fieldValue({00024}, issuesUnderEpic() UNION linkedIssues("",issuesUnderEpic()))) /480