The way I determine legs after the opening weekend is to divide the total amount a movie makes from a certain weekend forward by the latest weekend gross. For example, IM3 got $19,324,592 on its 4th weekend, and it made $60,909,748 from the start of that weekend to the end of it's run. That would be a 3.15x multiplier off the 4th weekend. When comparing it to GotG2, it would be safe to say it should have at least that same multiplier going forward, because it has shown smaller drops so far. I can then use that multiplier to find out how much GotG2 would make if it had that same multiplier after its 4th weekend.
But since the numbers are already so close to eachother, I can compare the absolute numbers to determine how much more it would make. The $60,909,748 IM3 made is pretty close to the (3.15*20,907,121=) $65,897,768 GotG2 would make with that same multiplier, and since I was trying to set a absolute minimum, choosing the lower IM3 number gives me a $5M buffer zone just in case does end up getting a lower multiplier than IM3 got from it's 4th weekend forward. So (60,909,748+313,315,927=) $374,225,675 would be the absolute minimum based on these calculations.
(And as you can see, if you use the multiplier, inflation won't change a thing, because it won't change that multiplier)
Yeah, you can see the release schedule here:
http://www.imdb.com/title/tt3896198/releaseinfo
As you can see, Japan and Nepal were the last 2 markets, and it opened there on May 12th. It was released in every other market at least 1 week before that.