Apologies if you've already tried an discounted the below method.
I'm not sure if this will work for your true setup, but I think you could try
manually dividing the sum of sales by the number of weeks, something along the lines of:
( DATEDIFF('week',WINDOW_MIN(MIN([Datefield)),WINDOW_MAX(MAX([Datefield])) )
could also be done with level of detail equations.