To know the difference in weight underwater for various cylinders, go to
http://www.oneandoneis2.demon.co.uk/DiveWeb/cylbuoy.xls for an Excel calculator
(Plug plug ;))
The absolute minimum lift a BCD must supply is the weight of air the cylinders can contain - since being correctly weighted means being neutral with empty cylinders with no air in the BCD. Extra buoyancy is for things like mouth above water, excess kit, excess lead to keep you warm, etc.
A 12L cylinder holds about 3.5kg of air, so the absolute minimum lift you need is 7kg - so you have 11kg surplus lift to use as you see fit
