i try to use multiple fuel sources to smelt exactly what i have. with 11 items, i slip in a piece of coal, and two planks. with 5 items, i use a sapling/stick first and three planks. the fuel order usually depends on whichever quantity of fuel is lowest, and i try to only use two and have one number as 1, so i can put in one type of fuel, then the second and walk off and program my turtles some more.