Forecast based on latest 26 weeks data
 

I am trying to create a weekly forecast (for the future 26 weeks) of sales
units based on the last 26 weeks of sales performance. This forecast (linear)
is updated on a weekly basis as the weeks sales come in as well as the
"last_valid_week" named range. I can not get the starting point of the offset
function to work correctly. Here is a sample of my formula(that doesn't work).

=FORECAST(BL$3,OFFSET(ADDRESS(ROW(),MATCH(Last_val id_Week,3:3,),1,,"sheet1"),0,0,1,-26),OFFSET(ADDRESS(3,MATCH(Last_valid_Week,3:3,),1 ,,"sheet1"),0,0,1,-26))
Row 3 contains the date values and the equivalent row number(Sheet1) to the
current row(Forecast sheet) contains the sales data.