Sub YourMacro()

columnLetter = ColLtr(ColNum("Z") + 10)
MsgBox columnLetter

End Sub

Function ColLtr(iCol As Long) As String
    If iCol > 0 And iCol <= Columns.Count Then
        ColLtr = Evaluate("substitute(address(1, " & iCol & ", 4), ""1"", """")")
    End If
End Function

Function ColNum(iCol As String) As Long
    ColNum = Range(iCol & 1).Column
End Function