The amount of damage Living Inferno will deal is locked in as you activate its ability, since you're dividing damage. If Living Inferno's power changes after its ability is activated but before it resolves (via Giant Growth, for example), the amount of damage Living Inferno deals won't change. The same is *not* true for the amount of damage dealt to Living Inferno by the other creatures, which is determined when the ability resolves.

You divide the damage among the target creatures as you activate Living Inferno's ability. Each target must be assigned at least 1 damage.