you can sign up tot he fertility friend website and there you can enter you temp each morning so you can see exactly when you ovulate and can then time BP a bit more acuratly, plus you can see from your temps if its likely that your PG cause otherwise your temps start to drop off before AF.
There is this belief that women with a 28 day cycle ovulate on day 14 when this is by no means true, so it may be that your Bding at the wrong point. If you head over to the TTC section and the chart stalker thread, there will be loads of other charters there who will be happy to help you out.