Average formula for a rolling range?

Hello,

I am trying to create a formula that looks for the last populated cell, counts 12 cells back and returns an average for all of the cells in that range. I know I have done this before with some combination of INDEX and COUNT, but I can't seem to remember how I did it and my experimenting is not proving fruitful.

Attached is a simplified version of the layout I am working with

Any help is much appreciated.

Thanks!