Specific running total column - next n rows per level

[Mateusz_Płonka]

Hi I need to include per row sum of column for next 12 indexs, but I have problem with my formula. 
Somebody know the solution? 
See me screen with description of result I need - probably need to fix conditionals but don't have idea how to do it.

Demand 13 Column =

CALCULATE(sum(DemandWeekly[DemandWeekly]),

                ALLEXCEPT(DemandWeekly,DemandWeekly[KeyDemandWeekly],DemandWeekly[ProductCode]),

                DemandWeekly[GroupIndex]>=EARLIER(DemandWeekly[GroupIndex]) && DemandWeekly[GroupIndex]<DemandWeekly[GroupIndex]+12  )

[MarkLaf · Accepted solution]

The relatively new WINDOW function is probably the best approach for this. 

Demand 13 Column = 
VAR _window = 
    WINDOW(
        0, 12,
        ORDERBY( DemandWeekly[GroupIndex], ASC ),
        PARTITIONBY( DemandWeekly[KeyDemandWeekly], DemandWeekly[ProductCode] ) 
    )
RETURN
CALCULATE( 
    SUM( DemandWeekly[DemandWeekly] ), 
    REMOVEFILTERS( DemandWeekly ),
    _window  
)