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procedure TForm2.HttpWork(ASender: TObject; AWorkMode: TWorkMode; AWorkCount: Int64);
var
  Http: TIdHTTP;
  ContentLength: Int64;
  Percent: Integer;
begin
  Http := TIdHTTP(ASender);
  ContentLength := Http.Response.ContentLength;

  if (Pos('chunked', LowerCase(Http.Response.TransferEncoding)) = 0) and
     (ContentLength > 0) then
  begin
    Percent := 100*AWorkCount div ContentLength;

    Image1.Width := round((245/100)*(50+0.5*Percent));            // funktioniert nicht
    Memo1.Lines.Add(IntToStr(round((245/100)*(50+0.5*Percent)))); // funktioniert
  end;
end;