The mathematical relationship is a little complicated (hence the reason for the calculator), but the rules of earning Arena Points aren't real hard to figure out-- every week, you earn points according to the highest team rating you've got. And a higher rating on 5v5 is worth more than 2v2, because 5v5 teams are harder to both fight and keep up with. This leads to a little bit of system gaming (and a lot of team jumping), but so far Blizzard has been fine with all of that-- they want 5v5 to become the most rewarding type of arena match, and they're willing to accept that you can often earn more playing 5v5 than 2v2, even if you lose.
Unfortunately, the actual Arena Rating system is a little more complicated-- it's based off of a chess rating system called ELO (named after the guy who made it, Arpad Elo), and the rating of your opponent actually determines how your rating changes as you play. Unfortunately, with no way to tell who your opponent is before you play a match, it's extremely hard to figure out your rating depending on how many matches you play (players are generally saying that the fewer matches you play, the better, as the higher your rating gets, the more difficult opponents you face). But of course the best way to come out with both a great rating and lots of points is to, y'know, actually be good.