Yes, the higher you set the damper, the less it will open and it draw more heat out of the stove, that regulates the draw on the stove and keeps more heat in the stove once setup correctly. I would leave it at .04. With coal, you want lower stack temperatures, that means you are getting the most heat out of your stove. Unlike wood, where you have to run hotter, you don't have any buildup of anything but fly ash, which is non-combustible anyway. You will notice when it gets colder, the damper may open even more, since the hot flue gases will rise much faster.
Sounds like you have getting it dialed in, my CoalTrol is set 6 min and 47 max and seems to work really good.
You may want to invest in a Dwyer Draft Gauge (mark II) just to verify and maintain your draft accurately.