Simple DAX forecast distribution with seasonality: https://wp.me/p6lgsG-2tS

Forecast_Seasonality_1.dax

SumForecast = 
VAR _previousMonth =
    CALCULATE (
        MAX ( 'Date'[Year Month Number] ), -- Must be the column that is connected to the Forecast-table
        REMOVEFILTERS ( 'Date' ),
        'Date'[Date] = TODAY ()
    ) - 1
VAR _latestYTD =
    CALCULATE (
        [YTD SumSales],
        REMOVEFILTERS ( 'Date' ),
        'Date'[Year Month Number] = _previousMonth
    )
VAR _cumulPercentage =
    CALCULATE (
        [YTD SumPercentage],
        REMOVEFILTERS ( 'Date' ),
        'Date'[Year Month Number] = _previousMonth
    )
VAR _amountForTotalYear =
    DIVIDE ( _latestYTD, _cumulPercentage )
VAR _result =
    SUMX (
        'Forecast',
        VAR _isPlanningMonth =
            ( CALCULATE ( MAX ( 'Date'[Year Month Number] ) ) > _previousMonth )
        VAR _relevantYTD =
            IF ( _isPlanningMonth, _amountForTotalYear * Forecast[ % Forecast], [SumSales] )
        RETURN
            _relevantYTD
    )
RETURN
    _result