Jira automation has an event "Issue transitioned to Done", but there is no way to write logic in this event. It allows is updating values of other fields, etc.. Our need is to iterate through all the epics in Milestone, add up all the % progess and divide by the number of epics. Then update the Milestone Issue Progress% field with this calculated number.
We use Structure, which correctly does exactly this.
But then there is no way to pass the % Progress value of Milestone in the Structure to "Progress%" field in the Milestone Issue.