Calculate monthly average in pivot table

Hi there!
I am attempting to do something that I'm sure is really simple, but I just can't figure it out.
I have a table of sales data (the source is a power query)... and have created several pivot tables from these data.
One of these pivot tables shows each service in Rows, and transaction dates (grouped by months and years) in Columns. The Values are sum of qty of sales (units) for each service in each month.
Attachment 753887

When I change the Value Field Settings / Summarize value fields by to Average, the pivot table calculates the average of units in each month rather than the average of each of the months:
Current average.png

What I would like to do is to add an column (ideally in the same pivot table as Sum of Units) that contains the average monthly sales of each service.
Attachment 753888

I would really appreciate any help in figuring this out... I'm sure I've got a facepalm incoming!

Thanks in advance!