A Fuelcan starts at 0 fuel (I know ... duh).
A Coalfuel Cell will add 12740 EU to the accumulated total value of the fuel.
A Biofuel Cell will add 4340 EU to the accumulated total value of the fuel.
Gunpowder will multiply the current accumulated total of the fuel by 1.4.
Glowstone will multiply the current accumulated total of the fuel by 1.3.
Redstone will multiply the current accumulated total of the fuel by 1.2.
You'll note that because of this, it is always advantageous to put your Cells into the can first, then your additive dusts afterwards.
Examples of how to calculate fuel value
yields:
:Coal Cell: 12740 (+12740)
:Coal Cell: 25480 (+12740)
:Coal Cell: 38220 (+12740)
:Gunpowder: 53508 (x 1.4)
:Gunpowder: 74911 (x 1.4)
:Gunpowder: 104875 (x 1.4)
while yields:
:Bio Cell: 4340 (+4340)
:Gunpowder: 6076 (x 1.4)
:Bio Cell: 10416 (+4340)
:Glowstone: 13541 (x 1.3)
:Bio Cell: 17881 (+4340)
:Redstone: 21457 (x 1.2)