To break it down a bit further:
You started at 15 build. You donated some stuff and got enough goblins to blanket the month of your first game and the three prior months, which is as far back as you're allowed to go. You then attended the Chicago April event. Assuming you have enough goblin points, you would then blanket for May, and that's the build you will have going into the next event. That's the order you want to use when you put it into the calculator.
To clarify, our typical weekend games are counted as two days, not three. I know it runs from Friday to Sunday, but it's less than 48 hours total.
Start -> 15 build, 0 loose exp
January blanket -> 20 build, 0 loose exp
February blanket -> 25 build, 5 loose exp
March blanket -> 29 build, 2 loose exp
April blanket -> 33 build, 3 loose exp (this is what you had at your first game)
April 2-day event -> 39 build, 7 loose exp
May blanket -> 42 build, 10 loose exp (this is what you will have going into the next event)
May 2-day event -> 48 build, 4 loose exp (this is what you will have after the next event)
I'd really suggest using the calculator that phil90 linked above. Calculating it manually is generally more trouble than it's worth.