Sub X()
    Dim ws As Worksheet
    Dim rngFirst As Range
    Dim rngLast As Range
    Dim rng As Range
    
    For Each ws In ThisWorkbook.Worksheets
        For Each rng In ws.Cells.Columns( 1 ).Cells
            If Not IsEmpty(rng) Then
                Set rngFirst = rng
                Exit For
            End If
        Next rng
        
        Set rngLast = rngFirst.End(xlDown)
        Set rng = ws.Range(rngFirst, rngLast)
        rngLast.Offset( 2 ,  0 ).Formula = "=Sum(" & rng.Address & ")"
    Next ws
End Sub