Devide by one time in a multiple values

Hi,

 

Hi,
I have a time table with days_in_month field to count everyday in a month into a column and the value for this field is the same for each row belongs to a month.
Now i want to create a new measure by divide Total Sales / days_in_month but just only one time instead of dividing 31 times for Mar.

Assuming Total Sales for Mar = 1M$, now i want to create this measure(named avgSales) = Total Sales/31

Could anyone help me? thanks in advanced

Best regards,
J.