A good design uses the center targets sparingly, high risk/reward shots. TNA is a good modern example. I mostly hit the 3 center (keypad targets) either during Multiball or when the ball save timer is running. This changes the strategy of the game toward earning multiballs and ball saves so the keypad can be safely completed and the reactor advanced.

In TNA You have to “complete” the 3 midline “keypad” targets to start the reactor. The entire game has you repeating this nine times if you are good enough to make it that far. But you don’t need to have the reactor running to activate a ball save timer or get a multi ball.