You may be close to your ideal position depending on how cold out it is. I would personally lean toward using the air control on the stove mostly; meaning open the damper a little at a time and get to where you can fully close the air control. Note that the colder it is outside the better the draft, so get the damper set on the coldest days you're likely to have and be aware that when it's less cold out you might have to let the primary open 1/8-1/4.
The bigger the vent, the more air will move; so the biggest you can manage without being too inconvenient.
The bigger the vent, the more air will move; so the biggest you can manage without being too inconvenient.