Rule 2 of 25 · Chapter I — Before You Move
Place the ball roughly two ball-widths ahead of centre for a short iron and move it forward toward the lead heel as the clubs get longer.
Why this rule exists
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.
In practice
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.
When it doesn't apply
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.