Are you trying to get an average of all numerical values in the rows and columns? Are these different measures or one measure?
Where and how would you like to display it? I.e. would you like to repeat the value in each cell, or display one single number without the months and dimensions on the rows?
This is what I would like the results to look like for each row of numbers. This calculates a average for the total array of numbers.
Remove your dimensions off the row shelf, right click on your measure and select Measure > Average
Upload a Tableau Packaged Workbook including sample data and I'd be happy to help.
I can tell you now - you won't be able to display an average column like you have in your latest screenshot alongside your 'Months' and before the Total.