Needs to line up just about spot on with the ball of your foot. May not be the same position for each foot (it's not for me). You'll know it if you ride long rides because your feet will start bothering you if it's off, or maybe only one foot will (happened to me). When things are spot on there shouldn't be any pain in the feet, everything should feel efficient as far as power transfer, and your patella should align to the ball of the foot at the 6 o clock position.