Originally Posted by
douglas
You've got that wrong. The penalty is 20% for each pair of classes that is too far apart, not for each level that one class is off. Your fighter 6/bard 3 has a 20% xp penalty for just a single pair of classes too far apart. A fighter 3/bard 1/rogue 1/cleric 1 would have a 60% xp penalty, assuming none of those is his favored class, because he has three different classes that are each more than one level different from his fighter level.
PrCs were exempted from the penalty because they are supposed to be further specialization within an area, while the multiclass penalty is supposed to represent trying to generalize too much.