Opening the book…
The club reaches its low point somewhere near a fixed spot relative to your chest, so where you put the ball decides whether you catch it before or after that point. Longer clubs have a shallower approach and a wider arc, which wants the ball a touch further forward; wedges want it back so you can still trap it. Get this wrong and you produce thin or fat shots that no amount of swing work will cure.
Use a fixed reference — a shaft on the ground, or the logo on your shirt — rather than guessing, since ball position drifts without you noticing. Hit five 8-irons, then five 5-irons, and check where the divot starts in each case: it should begin just after the ball's original spot both times. Adjust in small increments, half a ball at a time, not whole inches.
Move the ball back of these positions for punch shots, hardpan, or when you need extra loft taken off. Bunkers and deliberate high, soft lob shots use a forward ball position with an entirely different low point.