If damage dealt by a source you control is being divided or assigned among multiple permanents and/or players, that damage is divided or assigned before adding 1. For example, if you attack with a 5/5 creature with trample and it's blocked by a 2/2 creature, you can assign 2 damage to the blocker and 3 damage to the defending player. Those amounts are then modified to 3 and 4, respectively.