Let me help you a bit then (I didnt purchase the script, but I can help you test it, if you need me to)
So, you only need half the coal to make steel bars, so coal:iron ore = 1:1
If you have a coalbag, you can keep an entire inventory of coal in the bag (27 pieces)
If you first drop the coal on the conveyor, you can then add iron ore (if you first do iron ore, you will get 27 iron bars instead of steel) but as you can't withdraw coal from your coalbag if your inventory is full of iron, you need to create a small inventory of 27 coal in the furnace first.
1. So you take out 27 coal on the first bank
2. put it on the conveyor
3. Take out 27 coal from the bank
4. Put it in the coalbag (Make sure you dont press the button on the bottom to confirm there's coal placed in the bag.. it currently does that and no sane person has ever clicked on that)
5. take out 27 iron ore
6. Put iron ore on conveyor
7. Withraw coal from coalbag (again making sure to not confirm the chatbox that you took out the coal)
8. put coal on conveyor
9. withdraw steel bars from hotplate
Repeat steps 3-9
With at the end, still 27 coal in the furnace, and assuming you took equal portions, only withdrawing iron ore for the last run. (so I think in your script you have to remember that you have coal in stock in the furnace.
For mithril you'd do the following: take out 27 coal, put it in coalbag, take out 9 coal, take out 18 mithril. In this case, as you cant make anything out of mithril ore without coal, it doesnt matter if you did not stock up the coal in the furnace.
I can imagine that with mithril, as you are not making 27 bars each run, it could be more useful to just fill up the furnace with coal, but by my understanding, the 18 mith bars method is still faster.
Again, I can help you out with some testing, just pm me