How do I knock out only the correct sized knock-out with Carlon A238 ENT boxes?
They're not concentric so this might be easier than you think. I would take two flat-head screwdrivers to do this. Hold the box with one flat head against the back so that the head is supporting the 3/4" and 1" knockouts. Use the other flat head to pry the 1/2" out.