Experiment and use what works best for you. The main objective is to pull the trigger rearward without moving the rifle. If you can do it by sticking your finger further past the first pad it is fine.
Shooting techniques are generally very traditional, but don't bew afraid to adapt to what works for you. I personally don't use perfect pad placement and tend to favor more toward the joint. It allows me a more natural position using the standard AR grip (which is neccessary for HP Service rifle) and gives me better leverage on the trigger. Again, the objective is to be able torelease the hammer without moving the gun.