All tamagotchi's within the connection era (And many vintage and color tamas as well) fallow the same rule of thumb that 3 pounds lost if you win the game, 2 if you are over half way, and 1 if the game is lost early on.
Another nifty trick that can also be kept in mind is that you have a decent chance of your tamas weight (And happy/hunger ; ) being set to the minimum, although this is only really needed if one's weight becomes greatly out of hand.