Macro - Create / Copy / Paste Formula every 5 rows for varying range

I don't know if you want to delete the empty rows but if you do you could add this to the code.
Make sure you try it on a copy of your workbook in case it is not what you want.

Range("L1:L" & lr).SpecialCells(4).EntireRow.Delete

Put this line between "Next i" and "End Sub"

My bad. I misread your requirements.
Try this instead. Again, on a copy of your workbook.
If you don't want the empties deleted, delete the line before "End Sub".

Sub Try()
    Dim lr As Long, i As Long
    lr = Cells(Rows.Count, 3).End(xlUp).Row
    For i = 3 To lr Step 5
        With Range("K" & i).Resize(, 2)
            .Formula = "=R[3]C[-10]"
            .Value = .Value
        End With
    Next i
    Range("L1:L" & lr + 3).SpecialCells(4).EntireRow.Delete
End Sub