Calculate forecast based on increase and previous calculated month result

LostInTheFlood,

Try these measures. I made the Period column a date by adding the first day of the month (2022-01 becomes 2022-01-01). This will enable you to create a relationship with the date table. Alternatively, you could create a Period Number column with values like 202201, and use Period Number instead of Period in the Running Total measure.

Actual = SUM ( FactTable[Actual] )

Forecast = SUM ( FactTable[Forecast] )

Running Total = 
VAR vLastActual =
    CALCULATE (
        LASTNONBLANKVALUE ( FactTable[Actual], [Actual] ),
        ALLSELECTED ( FactTable )
    )
VAR vResult =
    IF (
        MAX ( FactTable[Actual] ) <> BLANK (),
        [Actual],
        vLastActual
            + CALCULATE (
                [Forecast],
                FILTER (
                    ALLSELECTED ( FactTable ),
                    FactTable[Period] <= MAX ( FactTable[Period] )
                )
            )
    )
RETURN
    vResult

LostInTheFlood,

Try this measure. I adjusted the DAX to handle months where both Actual and Forecast exist.

Running Total = 
VAR vLastActual =
    CALCULATE (
        LASTNONBLANKVALUE ( Actual[Actual Value], [ActualValue] ),
        ALLSELECTED ( Actual )
    )
VAR vLastActualDate =
    CALCULATE (
        LASTNONBLANK ( 'Date'[Date], [ActualValue] ),
        ALLSELECTED ( Actual )
    )
VAR vResult =
    IF (
        MAX ( Actual[Actual Value] ) <> BLANK (),
        [ActualValue],
        vLastActual
            + CALCULATE (
                [ForecastValue],
                FILTER (
                    ALLSELECTED ( 'Date' ),
                    'Date'[Date] <= MAX ( 'Date'[Date] )
                        && 'Date'[Date] > vLastActualDate
                )
            )
    )
RETURN
    vResult

Now you can take that vacation. 🙂