Average of Values in Matrix Visual created by Measure

Hi everyone,

I have a matrix visual which is populated by a measure which simply calculates the difference between a sum of data in one month and the previous month:

The measure is calculated as follows:

The measure is evaluated across several months in the visual and behaves as expected.

What I want is to average these values across the periods (which might be filtered by a slicer).

So I would expect the average for London to be: -2899 and Thames Valley to be: -1266

The source data is organised as follows where backlog date is always the first of the month and backlog_date 2 is always the previous month (also first of the month):

I have created the following measure to attempt to average the values to find the "typical month on month change":

but it does not produce the values as above.

Please could somebody assist with what I'm doing wrong? 🙂