Opening the book…
Iron distance is decided as much by strike location as by swing speed — a groove or two toward the toe or high on the face costs yards you will never diagnose from ball flight. Feel is unreliable here: most players cannot tell a centre hit from one half an inch out, and gear effect masks it. Spray turns an invisible variable into a picture you can act on.
Dust the face with foot spray, chalk, or dry-erase marker and hit five to ten shots, then read the pattern rather than any single mark. You want a tight cluster around the centre and slightly below it, where iron faces transfer energy best. If the pattern sits high, check your low point; if it drifts toward heel or toe, check ball position and distance from the ball before touching your swing.
Spray marks on the face are for practice, not competition — check your local rules before applying anything in a round. And ignore the pattern from a handful of swings if the lies were uneven or you were working on something new; you need a stable sample to trust it.