How to access row and columnvalues in a TfrxDBCrossView

explido

my intention is to have the "grand total" column containing two Memos instead of only one.
the column should not only contain the summed total but also another value calculated from the cellvalues of the current row.
i did not succed in implementing a second memo into the "grand total" column instead i created a new TfrxMemoViews beside each cell of the "Grand Total" column.

the main problem now is that i want my new memo to contain a value calculated from the previosly printed cells in the corresponding row.

the ColumnIndex of my "Grand Total" Column is 3 i can acces the rowvalues[0] but trying to access rowvalues[1] results in an index out of range exception
i would like set the value of the new collumn is the example below as follows:

text := rowvalues[ColumnIndex - 1] - rowValues[ColumnIndex - 2];

procedure DBCrossUmsatzOnPrintCell(AMemo: TfrxMemoView; RowIndex, ColumnIndex, CellIndex: Integer; RowValues, ColumnValues, Value: Variant); 
begin    
  if DBCrossUmsatz.IsGrandTotalColumn(ColumnIndex) = true then
                                          
    with TfrxMemoView.Create(AMemo.Owner) do begin
        left :=  AMemo.Left + AMemo.Width;
//        text := IntToStr(RowIndex) + ' - ' + IntToStr(ColumnIndex) + ' - ' + IntToStr(CellIndex);                                                        
        text := rowvalues[ColumnIndex - 1] - rowValues[ColumnIndex - 2];
        parent :=  AMemo.Parent;
        Width :=  AMemo.Width;
        Height :=  AMemo.Height;
        Top :=  AMemo.Top;
        Frame.Typ := 15;                                          
     end;                                   
end;