from pg 1 of the Koker manual:
Draft and air intake settings are preset at factory and it is usually not necessary to change for installation going straight out through the wall. It is still recommended to check draft settings with draft gauge after starting fire.
My understanding is the draft orifice is preset at the factory, The control rod is locked in place with a set screw and a dot of white paint is applied to mark the rod position. Your draft will vary with different conditions but should always remain on the negative (drawing) side. As the season goes on you'll find you do need to clean the flyash out of the fan housing to maintain a good draft. I'm told "as a rule of thumb"a cleaning is required for every ton of coal burned.
On my Koker, I opened the draft orifice all the way and locked it there. I then put a rehostat on the vent motor to contol the speed. Now I can dial in the exact draft I want. I also have my manometer permanently mounted so I get a constant draft reading.
I think you'll find if you leave the draft control rod at the factory set position and keep the fan housing cleaned out, your draft will be sufficient.