Opening the book…
Approach shots are decided by where the ball lands, not where it stops, and greens rarely give you the same roll twice. A yardage built from your best-ever strike is a number you only reach when everything is perfect, so it makes you come up short far more often than long. Clubbing off your comfortable carry means your misses cluster near the middle of the green instead of in the front bunker.
Hit ten balls with each iron at a swing you could repeat all afternoon, throw out the two worst and the best, and record the carry of the rest. Use a launch monitor if you have one; if not, pace off a well-struck shot's pitch mark on a soft range or fairway. Write the numbers on a card or in your phone and treat them as your working set until you retest.
On a hard, dry links where the ball runs a long way, you may plan for total distance instead — but only once you've chosen a landing spot on purpose. Retest after a real swing change, a shaft change, or a season away from the game.