"So what happens if you have a weak, queenless hive that you need to combine and you just place the box on top of the strong hive? Is there a kind of "magic" number that you need to shoot for to newspaper combine? "
tbone, there is no 'magic' number. i would suggest you use the newspaper method to combine any queenless colony with a queen right colony.
there is a great difference between 'equalization' of colonies and 'combining' colonies. equalization of colonies consists of going through your beeyard, and determining which hives need a 'boost', and which hives are the strongest to give the weaker hives the boost; frames of brood and nurse bees, and/or pollen and honey, and equalizing all....similiar in strength and stores. nurse bees have no alliance, so typically one is safe to transfer these bees and the brood they are caring for without fear of the loss of your queen or bees fighting. not so with combining a hive of bees with another.
in a combine, it is safer to use the newspaper method to ensure that your queen is not injured/killed and/or that you don't have a boat load of bees fighting and killed, and there is no amount of smoke or syrup sprayed on them that will prevent this. combining bees is stressful on the bees; to the stronger hive you are combining them with, and the weaker queenless hive you are giving a 'new home'. the newspaper method gives them the time needed.
considerate of the bees? yes, maybe, a little more to it than that. if you are not 'considerate' of your bees, perhaps one shouldn't be keeping bees.
that's my humble opinion.