Good Afternoon

I'm having a little problem with summery filters. I've a two view workspace, one that lists items and the other display sales quantities. The Month-Year column is a calculated column which is sorted and summarized. This gives me the sales totals by month as well as total sales (I have a query date filter set up so I can select various date ranges). What I would also like to see is the average sales per month (411/12). However anything I've tried is not working. Any help would be appreciated.

Thanks