I believe you're going to need a LOD calculation. Attached is a picture and workbook.
let me know if this is the answer you needed, it recreates your numbers. Not sure what the end result you want is.
great video on LOD here: Think Data Thursday: LoD of the Rings - YouTube
AverageofAverages.twbx 9.9 KB
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If you are ok with having the column totals for your number of records and total usage, you can change the aggregation of the "Per User Average" field.
1) Analysis -> Total -> Show Column Totals
2) Click on the total for "Per User Average" (should be the 146.7)
3) Click "Automatic" and change the aggregation to "Average". This should turn it over to the 123.3 you are looking for.
Matthew's answer was what I was looking for, as I'd like to be able to save the calculation and plot the value on separate charts. I'll research the "fixed keywords" a bit more and watch the video you linked on LoD.
Eric's answer was also helpful for showing the average of averages as a grand total in the table.