Multiplication with Previous Values

sam_hoccane 
Your wishes are my command 😉 Here is the workable solution exactly as you wish 
https://we.tl/t-bo914kjbfJ

Predicted_Measure_4 = 
VAR CurrentEmission = SUM ( Data[Emission] )
VAR CurrentMonth = SELECTEDVALUE ( data[Year_Month] )
VAR CurrentForcast = SELECTEDVALUE ( Data[forcast] )
VAR PreviousMonths = FILTER ( ALLSELECTED ( Data ), data[Year_Month] < CurrentMonth )
VAR PreviousN_Months = FILTER ( PreviousMonths, Data[forcast] = "N" )
VAR PreviousN_Month = MAXX ( PreviousN_Months, data[Year_Month] )
VAR PreviousN_Emission = SUMX ( FILTER ( PreviousN_Months, data[Year_Month] = PreviousN_Month ), Data[Emission] )
VAR YearMonthOnly = DISTINCT ( SELECTCOLUMNS ( PreviousMonths, "@YearMonth", Data[Year_Month] ) )
VAR NumberOfY_Months = COUNTROWS ( FILTER ( YearMonthOnly, [@YearMonth] > PreviousN_Month && [@YearMonth] <= CurrentMonth ) ) + 1
VAR Result =
    IF (
        CurrentForcast = "N",
        CurrentEmission,
        PreviousN_Emission * POWER ( 1.15, NumberOfY_Months )
    )
RETURN
    Result