Capstar comes in two forms:
Capstar for cats and small dogs up to 25lbs containing 11,4 mg nitempyram
Capstar for large dogs over 25 lbs containing 57 mg nitempyram.
Capstar tablets are administered orally at a minimum recommended dose of 1mg/kg (0.45mg/lbs). At your dogs weight of 12 lbs it needs a minimum of 5.45mg. A tablet of Capstar 11,4mg would be enough. If you have the tablet of 57 mg, even if you cut it in four it would be too much anyway but if we consider that this is one of the safest medications against fleas that would be acceptable. Bear in mind that the risk of side effects is higher when you administer an overdose.