It also depends on the lofts of the irons I'm playing that day. I recently switched irons and lost one club, or so I thought. But it turns out the lofts are similar between the 5-iron of my current set and the 6-iron of my old set.
I'm literally getting a distance number, thinking in my head, "I would hit a 6-iron here, so I need a five," and then pulling the 5-iron for said shot. Lets say that distance is 165. Do I hit a 6-iron 165, or do I hit a 5-iron 165? The answer to both questions would be "yes."