Custom Furnace smelting 1 item less

Hi!
I wanted to make 3 custom furnaces, taking less time to smelt with each one, while maintaining proper fuel consumption.
So almost everything works as intended, but the last item smelt.
For some reason this last one always stays in raw form. For example I fed my furnace 1 coal and 8 cobble, got 7 stone back. Fed him 2 coals and 16 cobble, got 15 stone back.
Ticks for fuel are 800 and smelt time for any item is 100, yet it still doesnt work.
Appreciate any help