cursive calculation forecast

This might work:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQwMFCK1QEyjGAMUyjDCCalgI2MBQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Sales = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Sales", type number}}),
    AddIndex = Table.Buffer(Table.AddIndexColumn(#"Changed Type", "Row", 1, 1)),
    ListGenerate = List.Generate(()=> 
        [Counter=1, Forecast=AddIndex[Sales]{0}],
        each [Counter] <=Table.RowCount(AddIndex),
        each [Forecast = if AddIndex{[Row=Counter]}[Sales]<>null then [Forecast]*0.7+AddIndex{[Row=Counter-1]}[Sales]*0.3 else [Forecast],
        Counter = [Counter]+1
        ]
    ),
    Forecast = Table.FromRecords(ListGenerate),
    #"Merged Queries" = Table.NestedJoin(AddIndex,{"Row"},Forecast,{"Counter"},"Forecast.1",JoinKind.LeftOuter),
    #"Expanded Forecast.1" = Table.ExpandTableColumn(#"Merged Queries", "Forecast.1", {"Forecast"}, {"Forecast"})
in
    #"Expanded Forecast.1"