Hello everyone !
To give you more information on my instance and context :
I'm on Jira Cloud Premium, I have setup an Issue type hierarchy with an Initiative level on top of Epic.
I have added the custom field "Parent Link" to my Epic.
Mostly, all the link are being made through Advanced Roadmap with "Review changes".
So 1 Initiative --> Multiple Epic --> Multiple Stories ...
Now the question :
I have in my Epic a numerical field that is a % of progression of advancement which is added manually (% Progression)
I'm trying to calculate the average of this field and add it to my issue type Initiative custom field "Sum of Progression"
I know the formula in math (lol) : (Sum of Progression) / (Total number of Progression) = %
but I don't know how to formulate it in Jira smart value / variable / ... 
The easy part of the Automation I got :
Trigger : Value changes for % Progression
Issue field condition : Issue type = Epic
Then lookup issue : "Parent Link" = {{issue.parent.key}}
If anyone could please give me a hand on this, or even brainstorm with me it could be super great and really helpful !
Have a nice day !