Answer
Don't know the answer for N boxes. For two (depending on decks and rules, of course), it is W-W: .30, L-L: .31.
For the rest, in case you're interested, it's:
W-L: .24
W-P(ush): .04
L-P: .10
P-P: .01
By the way, these values come from SW's Tournament Blackjack.
Don