dax to forecast based on same period 4 previous fiscal week

[misterMethod]

Need help wirting the dax equation to to farecast based on YoY values. 
I need to forecast the sales bases on same period last year ( custom fiscal calendar can't use time intellingence function) . 
Let say  we are in  fiscal week 10 and I need to forecast all values from week 11 on. 
Forecast sale :
                             current_week sale + average 4 YoY sales (starting week 11 last year  till week 7 last year) 
YoY = CY_Sale - PY_Sale
CY Sale = calculate(SUM('sales'[sales Amount]), fiscal_week <= Current_week)
PY_Sale = calculate(SUM('sale'[Sales Amounr]), 'date'[year] = prev_yr)



  

[v-yadongf-msft]

Hi @misterMethod ,

Can you share some data screenshots to help us beteer solve your problem (remember to hide sensitive information)?

Thanks for your efforts & time in advance.

Best regards,

Yadong Fang