I am trying to calculate a cumulative average column on a table I'm making in Excel.

I use the totals row for the ending cumulative average, but I would like to add a column that gives a cumulative average for each row up to that point. So, if I have 3 rows I want each row to have a column giving the average up to that row and then the ending cumulative average in the totals row.

Right now I can't figure this out because I'd be having to reference in a formula rows above and below the current row and I'm unsure about how to go about it because it's a table and not just cells. If it was just cells then I know how to do the formula and copy it down each row, but being that the formula I need depends on whether or not a new row in the table is added or not I keep thinking that my formula would be something like:

(Completion rate row 1/n) where n is the number of rows up to that point, here row 1, then ((Completion rate row 1 + Completion rate row 2)/n) for row 2 so n=2, and so on for each new row added.