Because Fog Bank has 2 toughness, your opponent can assign a maximum of 8 damage to you.
For combat damage assignment, the definition of "lethal damage" is just based on a creature's toughness and any damage it already has marked on it (and whether or not the attacker has deathtouch). It doesn't take into account any effects that would prevent, reduce, increase, or otherwise change the amount of damage that would actually be dealt after being assigned.
April 18, 2016 1:16 p.m.
To that point, it also doesn't consider the actual outcome of damage. Trample damage assignment doesn't look into the future to determine whether the blocking creature actually will be destroyed. Indestructible in particular doesn't nullify trample.