Mates!

As far as my fixation goes for solo running large furnaces - it has rational explanation. You see - to smelt precisely 3,000 (or 3k, as some prefer) You need 1,250 wood. No more, no less - that means 3 stack EVENLY divided into 250 (which will take 12 spots), 1,250 wood (2 spots) leaving furnace with 4 slots - 3 for smelted ore, be it metal or sulfur, and one for charcoal (as some of You might not realize - is NOT burning 1:1 ratio from wood).

So, as I've asked before - either leave ore to smelt in one of the boxes, or follow this tiny instruction on how to EFFICIENTLY use large furnaces.