Average a series of numbers which occur every 4 or 5 rows

I've created a spreadsheet which tracks my weekly mailing list details (recipients, opens, and clicks). One column contains a cell with a forumula which subtracts the recipients at the beginning of the month from the recipients at the end of the month; giving me growth for that month (see below). Now I'd like to average the last X values to determine what my average monthly growth is.

If I were to describe it in simple terms:

• Offset to the end of the column, by say 16-20 weeks.
• Discard empty cells
• Take the last X values (since the count might occasionally include 5)
• Average by X

The problem is that the empty cells, and the fact that there might not be a value in the "last" cell in the column is throwing me off. Can anyone offer input into how I might go about this?

Here's a copy of my sheet for reference.

To average the last X values in column G2:G, use chooserows(), tocol() and sequence(), like this:

=let(
  X, 3, 
  data, chooserows(
    tocol(G2:G, 1),
    sequence(X, 1, -1, -1)
  ),
  average(data)
)

See let(), chooserows(), tocol() and sequence().