This seems simple enough, but averages sometimes have picky details that keep them from being quite so simple.
The immediately obvious answer is to define a summary field to compute the average of that calculation field and then place it in the sub summary layout part.
Summary fields placed in sub summary layout parts display results based on the set of records grouped with that sub summary layout part.
Picky details that can make averages more complex:
FileMaker Average summary fields and average aggregate function do not count missing values as zero. The average of 3, <empty>, 3 will be 3 (6 divide by 2), not 2 (6 divide by 3).
Averages sometimes need to be weighted averages
Thank you Phil - you've done it again! It has taken awhile for me to get back to this issue but am happy to say, with your suggested method, 'the report now works just fine'. The addition of a summary field, changes to the layout then minor tweeking took all of about 15 minutes!
The summary information now provided is powerful and yet zero additional work is now required. I've included an image of the resulting page....