So, i got my first batch of steel. What i did for this:
1) place 2 coffins into furnace and lit it at 6 am
2) after 7 days at 7 am my neighbor pulled coffins out of the furnace (to my great joy)
total burning time 7 days and 1 hour. At the same time i heated the furnace every evening and one day missed (fuel level drop at near 28%).
3) coffins lying on the ground one day and 6-8 hours near the fence.
As a result i got 8 blistersteel bars and no dross.