i have a mark3 in my basement that i heat a 24x40 ranch with a entry way. an outside chimney with a ss liner.the temp this morning outside was 2 above,i set the draft knob last night at 1 1/4 turn open,maybe a hair more,the stack temp was about 230(12"above elbow exiting the stove),the house is 72,13 hrs later.
i have found that anything above 300 degrees seems to go up the flue,i do have a baro with the weight backed all the way out(i dont get a strong draft),mostly works when the wind is blowing.
if the overnight temps are to get below zero,i usually only back the knob out no more than 1 3/4 turns,i work a swing shift and times cant tend the stove for 15 hrs.when i get home my house is the same temp it was when i left.i have had my stove going steady since the monday following thanksgiving. i hope this helps give you some idea on different things to try.