If it was just the average of all the records in the portal, this would be very simple. Average ( Portaltable::PortalField ) would return the average of Portal field for all related records in Portaltable.
Trying to limit this to just the "first 10" makes this much harder to do. We'd need either a relationship that matches only to those 10 records or a script that finds those 10 and copies the average back to a field on this layout before we could produce such a value.
What determines which records are listed first in these portals? Is there a specified sort order or are they listed in unsorted order (which is the order in which they were created)?