Sub stepAver()
myStep =  27      'шаг
myCol =  1        'столбец
begRow =  1       'начальная строка
endRow = ActiveSheet.Cells.SpecialCells(xlCellTypeLastCell).Row
isEnd = False
Do While Not isEnd
   With ActiveSheet
      Cells(begRow + myStep -  1 , myCol +  1 ).Value = _
      Application.WorksheetFunction.Average(Cells(begRrow, myCol), _
      Cells(begRow + myStep -  1 , myCol))
   End With
   begRow = begRow + myStep
   isEnd = (begRow >= endRow)
   If begRow + myStep > endRow Then myStep = endRow - begRow
Loop
End Sub