Can i use a footer total sum as a field?

Hi there, i was wondering how i can access the sum formula in a column footer as a field in a formula?  I have a column called "total paid" in my invoice table which gives me a figure of how much money i have invoiced and been paid from all clients.  I am trying to produce another table where i can see how much money i have made in the year to date but i cant figure out how to access the sum figure as a field.  Also i was wondering how i would be able to see this figure by selected month if possible.  Sorry for the noob questions