Originally Posted by
Sporeegg
It gets calculated the same as the Con bonus from rage. The main point is avoiding the "sudden death syndrome" induced by dropping under 0 while raging and suddenly loosing 2/3/4 * level HP due to falling out of rage.
Your average Con 16 -> Con 20 Barbarian on 3rd level looses 2 * 3 = 6 HP and if he drops to -5 before rage ends, you are effectively at -11.
Later on, your Con 18 (belted) -> Con 24 Barbarian on 11th level looses 3 * 11 = 33 HP. So when he falls unconscious on -1 HP, he immediately looses 33 HP, pushing him to -34 and killing him.