code: public function getSortValAfterAll($groupingId = null)
      {
          if (!$groupingId === null && $this->grpColumn) {
              throw new SortableException(
                  'groupingId may be omitted only when grpColumn is not configured.'
              );
          }
  
          $query = (new Query())
              ->select($this->srtColumn)
              ->from($this->targetTable)
              ->where([
                  'and',
                  $this->grpColumn ? ['=', $this->grpColumn, $groupingId] : [],
                  $this->skipRowsClause()
              ])
              ->orderBy($this->srtColumn.' DESC')
              ->limit(1);
  
          $result = $query->one($this->db);
  
          if ($result) {
              $result = array_values($result);
              $sortVal = $result[0] + $this->sortGap;
          }
          else $sortVal = $this->getIniSortVal();
  
          return (int)$sortVal;
      }
docs: Derives a sort value for a record to be inserted after all items.
  
  @param null|int $groupingId
  @return int
  @throws SortableException