david weaver
Well, those pictures and the caption are actually Ellis's contribution, so I'm assuming that he set his plane there. Maybe he can clarify.

But .004 or so is not too difficult to do. It's really just a tiny touch more than not being able to see the back of the iron glint back at you at all. It'll get to the point where you are accidentally probably setting an iron closer than that once you've done some repetitions of setting the cap iron as close as possible.

One thing we wanted to get away from is measuring anything, you only need to know the .004 number to know that it's .004 and not .04, etc. Your real hands-on approach should be to set it as close as you can and see what the results are. I guess it will depend on your eyes and light source, but if you set it as close as you can without spending several minutes to get there, you'll probably be in the ball park. You can forget, though, about trying to measure something that small if you follow that.

Then you can see what the shaving does, if it bunches up, you're too close. If you take a fairly coarse shaving and it still curls up nicely like a single iron shaving would, then you know you're probably not set close enough to have the cap iron doing anything other than protecting you from getting gigantic tearout if you accidentally set a deep cut when adjusting the plane.