Hrqls: the simple answer of 30% would be something along the lines of
*Needing to win 2 in a row games outright = 25%
*Increase this a bit due to the chance of a gammon or backgammon in the first game therefore not necessitating the play in the 2nd game hence an approximate 5% increase

I have no program for backgammon to work this out but I think this is a good approximation using the rule of thumb