You could script the refresh and then set the portal field(s) that will trigger the script when modified.
How to refresh that ? There are some refresh steps I tried Refresh Window , Refresh Portal Object etc ...Doesn't seem to work
Refresh window WITH "Flush cached join results" option checked will do the trick.
If clicking outside the portal forces a refresh, then do that in your script:
Go to Field [ ] // if you use this script step without specifying a field it is the equivalent of clicking outside of any field (or in other words, "go to no field")
Follow that with Refresh Window [ ] with Flush cached join results checked.
One or both of those steps should achieve what you want.
Nope ... Still not working
Both steps combined are working ! Thanks
clicking somewhere on the layout is the normal way to "commit" a FileMaker record.
I am assuming that if you click outside the portal then your calculation result shows correctly in the parent?
If this is the case then i suggest you force a "commit" when changing values in the portal.
Once the record is committed the unstored calculation should be updated and you should not need to refresh the window.
In theory you can do this by setting a script trigger to the relevant portal fields specifying: "On Object Modify". The script is a single step script: Commit record/request. [No Dialog]
However you will exit the field each time and have to click back in - The easiest would be to have a button in the portal called "update" or "calculate" - which just performs the Commit Record/Request single step script.
As for Refresh Window (which I don't think you need!) - here is a very interesting article all about refreshing windows:
Good luck and happy developing
Commit Record 
Set Field [ LayoutTable::primaryKey ; LayoutTable::primaryKey // the key used for the relationship to the portal TO ]
where setting (one of) the local relationship key(s) to itself recalculates the portal relationship only, and so is is less expensive than resetting all join caches.
I tried the commit record it wasn't working ... I will look up the link you added ... Thanks
Commit Records/Requests[No Dialog]
Refresh Window[Flush cached join results; Flush cached external data]
Flush cached external data is useless, unless you're displaying data from an external SQL data source and you want that data refreshed.