title: How to calculate a monthly average when the month is not finished
body: <p>Let's say I have a 12 cells on column A with the sales for each month last year. To calculate a monthly I can do a simple formula like this in A13 =sum(A1:A12)/12. But in column B I have the sales for this year, not a complete year, and August is not finished, but I have the running amount of sales for this month, so far. What kind of formula can I use to return a more accurate average?</p>
  
  <p>Eduardo</p>