Calling Oppo Scientists

Here's an interesting question for any science-minded Opponauts out there.

Since I have three growing boys, and since I don't want to go to the grocery store every other day for milk, I usually buy 12 gallons at a time and freeze it. Yes, you can freeze milk, you just have to shake it up since it separates when it freezes. Anyway, Costco used to stock milk that came in a plastic jug that had a depression on one side, and when the milk froze that depression would pop out as the milk expanded. Now they carry different jugs, and the depression is gone. About 20% of the jugs I freeze crack, and the milk drains out as it thaws.

Solution: pour a little milk out of the jug before freezing. But how much? Does anybody know how much to decant to allow expansion to occur without splitting the jug?