How to create an automation rule that updates an Epic's custom field with its completion percentage?

[Fernando Martinez]

I am trying to build an automation rule that meets the following requirements:

1. Executes weekly.
2. Selects target Epics via JQL.
3. For each of the Epics selected, the percentage completion of the Epic is computed. Completion of each child issue is defined as the issue being resolved.
4. For each of the Epics selected, the computed percentage completion is set into an existing field in the Epic.

I am not sure how to implement step (3). I was hoping to put the percentage completion into a smart value for each Epic, and then use that value in step (4).