DarwinKoala: I usually resign when I feel I've no chance left; not just with backgammon variants, also with other games. I don't go out of my way to calculate the chances, so I may play on for a move or two "too long", but whenever it's obvious, I resign.
And I greatly appreciate it if my opponents do so as well.
(hide) If you want to play a game with an opponent of a similar level, you can define a required BKR range for a new game invitation. Then nobody with a BKR outside this range will be able to see/accept it. (Katechka) (show all tips)