If I am taking my time, shooting S/A, I use the pad of my finger tip. A wide trigger is nice. For double action, I use the first joint. A narrow, polished trigger is best. If there are any rough spots, or inconsistencies in your guns action, your groups will suffer. A D/A pull can be heavy, but it should be smooth for best accuracy. Smooth, even, controlled pulls, not jerks, are what put shots on target. Back when D/A revolvers were our issue duty weapons, the saying on the range used to be "stroke the trigger, stroke the trigger". also, make sure you have a set of grips installed that fit your hand well for consistent grip and trigger finger alignment. Its hard to shoot D/A when the gun is moving around in your hand with each shot.
Larry