Source and target cells specification in a macro

[Mati44]

I have the following macro which works fine. I've been trying to specify the source cells (rows and columns) and the target cells(rows and columns) in the code but so far I was unsuccessful.

Sub mainx()
    Dim r As Range, c As Range
    With Cells(1).CurrentRegion
        With .Offset(1).Resize(.Rows.Count - 1)
            For Each r In .Cells
                Set c = .Find(r.Value, r, , 1, , , 2)
                If (c.Address <> r.Address) Then   
                    If c.Row > r.Row Then           
                        r.Offset(, 40) = c.Row - r.Row - 1
                    Else
                        Do
                            Set c = .FindNext(c)
                        Loop While c.Row = r.Row And c.Address <> r.Address
                        If c.Row > r.Row Then
                            r.Offset(, 40) = c.Row - r.Row - 1
                        Else
                            r.Offset(, 40) = "na"
                        End If
                    End If
                Else
                    r.Offset(, 40) = "na"
                End If
            Next
        End With
    End With
End Sub

The current code generates the results in the second row and after 40th column, and the code works on all the values until 40th column.
I need to make the code work on the data between A15:N100 (source cells), and output the results in AO15:BB100 (target cells). So that the values in between won't be affected by the code.

How can I achieve this, please? Can I get a little help? Many thanks!