For double action, the most accepted way is to have the outside edge of the trigger contacting the first joint of the finger. This is because the muscle advantage is more important than nerve sensitivity when overcoming a heavy trigger and keeping a steady gun.
For single action, move upto the mid to front of the finger pad. Some people have difficulty at first with this because the more extreme angle of the finger as it engages the trigger tends to lead to a "pushing" situation. But, once a person concentrates on the feel of the trigger and finger movement, that straight back "press" becomes easier and easier.