If you follow this method the end of your pieces will be as good as the start.
Once you have chosen you need to start your practice that many bars from the end of your piece. If you think you can learn 4 bars at a time then start your piece 4 bars from the end.
In your next practice session you need to start twice as many bars from the end of the piece (so if you thought you could learn 4 bars then you will now be starting 8 bars from the end).
The next day you start another 4 bars back (or however many you chose). Keep doing this until you work your way back to the start of the piece.