I run my Mark III at 2 - 2-1/2 turns with no problems. My stack temp runs between 200* - 300* F with a mag thermometer about 18" above the stove. As you said, depending on conditions (outside temp, wind, coal quality, how full the ash pan is, etc.) this may have to be adjusted a little. When I run it through the night, I will have it at 1 turn (got me from 10:30 last night to 7:30 this morning and there was still a very strong fire this morning), when I am kick starting it in the morning I will turn it all the way open for maybe 15 minutes while doing the coffee & dog chores, then I'll turn it back in.
I just had to replace a Mark III after 14 years because the heat baffle warped and cracked the sides of the stove. I had liked the first stove and got another Mark III to replace it about a month ago (easier to fit existing stove pipe configuration, too!). Since you said yours is used, I would check the condition of the baffle. My theory on mine is that the top of the baffle flaked away over the years making it thinner and less able to handle the temps inside the stove. I don't know if it was because of the chemical make up of the ash, moisture during the summer from being in an unfinished basement, or a combination, but it was definitely flaking and thinner toward the center of the stove.