For instance: I have 4 coals, each coal could smelt 12.5 items. But every time the fuel replenishes from the remaining coal the smelting process is reverted back to 0. I should smelt 50 items at total but since the process is reverted every time it replenishes I could only get 48 items smelted at total. Worse case is for fuels that could only smelt less then 1 item per fuel. It's always rounded down, so I get 0 instead.