( (Weight-of-Items plus Weight-of-Crucible) multiplied with (Target Temperature minus Environment or Current Temperature) ) divided by (500000 times the Burning-Box-Efficiency in Percent)
quoted from GT6 Basic Earlygame Stuff
The “times Efficiency in percent” means multiply with a number between 0 and 1, so 1 for 100% and 0.5 for 50%
And the Fuel Value its 5000 HU per Smelting Process the Fuel would give in a Furnace, so 2500 HU for a Sapling or 40000 HU for a piece of Coal. Note that the above Formula already factors in the Furnace Value.