Think you need to also create an Activity Custom Field which calculates the avg of the whole activity. That should then show up as an activity list column.
An Activity custom field is not the same as an Interval field! The first one displays in the activity summary at the top and is always for the whole activity. The second one displays as a number for a selection/interval header and as a column on the Intervals Data page.
Use your code to create a Custom Field here: