Issue with subtracting columns

If you are trying to add a column to the table with the value in each row of the last column subtracted from the value of each column in the nth row, you could do it like this... Just change the number in curly braces in the NthColumn variable to the column number that should be used.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WslDSUTIE4qLUFCBpqRSrAxEzAuKknNJUFEFjIE4vSk3NQxE1AeKC0qKCHFTFpkCcX5SYlw4VjgUA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, Column3 = _t, Column4 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", Int64.Type}, {"Column2", Int64.Type}, {"Column3", type text}, {"Column4", Int64.Type}}),
    add_difference = Table.AddColumn ( 
        #"Changed Type",
        "NewColumn",
        let 
            LastColumn = List.Last ( Table.ColumnNames ( #"Changed Type" ) ),
            NthColumn = Table.ColumnNames ( #"Changed Type" ){1}
        in
            each Record.Field ( _, LastColumn ) - Record.Field ( _, NthColumn )
    )
in
    add_difference