It is in the rulebook.
You receive xp equal to your build points. For example, if you have 25 build, and you attend a two day event, you get 25 xp per day, or 50 xp. At the next event, your build would be higher, thus your xp per blanket is more, but you need more and more xp to get build, so your progression slows down.
Page 50.