Nonononono... I am going from memory here, but as I recall, it's supposed to scale from the first 20 (a 20 after all modifiers) up one for each armor class step. So if the bad guy hits AC -2 on a 20, and he has an additional +2 to hit, he'll hit AC -2 on an 18, AC -3 on a 19, AC -4 on a 20, up...