In the end, it comes down to what works for you. Two fingers. Three fingers. If you are going to be taking high incoming or birds directly over-head, more pull will be better than less. When looking at photographs, understand that if the camera's shutter is pressed just as the shot is fired, recoil will move the gun back making pull appear shorter than as measured. You can check this out for yourself by mounting the gun and applying simulated recoil with your muscles.
If you bonk your beak, add pull.
If you bonk your beak, add pull.