A measure which shows the average weekly qty of products sold based on the last 12 weeks of sale

My objective is the following
 
from my 'sales' table I wish to create a column or measure which shows me the average weekly quanity of products sold based on the last 12 weeks of sales only?
 
my thought process was to filter the table to show only the last 12 weeks of data only.
Then to find the weekly average of goods sold I would sum the quantity and divide by 12
 
Is this a good route and would anyone know the DAX to achieve this??