It's not that simple, mate.
In Baldur's Gate, there was D&D 2nd ed. armor class. It was based on THAC0:
To Hit Armor Class 0. If you had a THAC0 of 2, it means that you needed to roll AT LEAST 2 to your hits to sucessfully hit someone with armor class 0. So if you had a THAC0 of -2 and the enemy had an armor class of -8 (I had -25 once), you had to roll at least 6 to hit him. 1 is always critical failure and 20 is always critical hit. This was easy to understand but the question is: if you have an armor class of 100, what does it block? A percent of the attack? If yes, what percent?
