Range starts from first cell of a dynamic column

I would like LastRow to start from the first cell of the input Column and count down.

What I had didn't work

Function AvgLastX(N As Integer, MyCol As String)
    Dim LastRow As Long

    LastRow = Range(Cells(1, MyCol)).End(xlDown).Row + 1
    
AvgLastX = WorksheetFunction.Average(Range(Cells(LastRow, MyCol), Cells(LastRow - N, MyCol)))
End Function

Please help,

NTB