Well, yes, I think so.

Also: Our family needs three gallons of water per day = Нашей семье нужно 12 литров воды в/на день (either one is possible).

ADDED:
It's like the basic option is a "В" but since it also means "INTO" you can't say INTO person instead of PER person. That's why it has been replaced with a "на". Just like with families. There's no "direction" (into a family) in "per family". So I guess that is the reason of this controversy. As for learning, yes, you must understand that you have to learn a lot by heart when it comes to all the bloody things about the Russian grammar. Just like about the grammar of any other language, including English with their complicated use of prepositions