Power Query: How to add a calculated row?

Hi kdecolve ,

We can use the following steps to meet your requirement:

1. group by the table based on the year column, save other as all rows

2. Create a custom column:

Table.InsertRows(
    [QuarterData],
    Table.RowCount([QuarterData]),
    {
        Record.AddField(Record.RemoveFields(Table.Max([QuarterData],"Amount"),"Quarter"),"Quarter","BestResult")
    }
)

3. expand the custom column exclude the year column

2. delete the useless column and rename other column:

All the queries are here: 

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjIwtFTSUXJMSgaSUZUVQNIQiC0MlGJ1sMoaATEuOWMgNsWp0wSIzeCyFlhsNcYpC7LV2BSXrDGSm9DlTKBuigUA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Year = _t, #"Extra info A" = _t, #"Extra info B" = _t, Quarter = _t, Amount = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Year", Int64.Type}, {"Extra info A", type text}, {"Extra info B", type text}, {"Quarter", Int64.Type}, {"Amount", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Year"}, {{"QuarterData", each _, type table [Year=number, Extra info A=text, Extra info B=text, Quarter=number, Amount=number]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each Table.InsertRows(
    [QuarterData],
    Table.RowCount([QuarterData]),
    {
        Record.AddField(Record.RemoveFields(Table.Max([QuarterData],"Amount"),"Quarter"),"Quarter","BestResult")
    }
)),
    #"Expanded Custom" = Table.ExpandTableColumn(#"Added Custom", "Custom", {"Extra info A", "Extra info B", "Quarter", "Amount"}, {"Custom.Extra info A", "Custom.Extra info B", "Custom.Quarter", "Custom.Amount"}),
    #"Removed Columns" = Table.RemoveColumns(#"Expanded Custom",{"QuarterData"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Columns",{{"Custom.Extra info A", "Extra info A"}, {"Custom.Extra info B", "Extra info B"}, {"Custom.Quarter", "Quarter"}, {"Custom.Amount", "Amount"}})
in
    #"Renamed Columns"

Best regards,