- Roll a d20, higher is better,
- ... But you can't go over your ability score,
- ... And an exact match is a critical success,
- ... But a 20 is a fumble.
If we insist on sticking to Dex, probably the best use for descending AC is not as a direct modifier to the chance to hit, but a modifier to damage. Not "damage reduction" (DR), per se, because that's a subtraction, which injects a little inelegance. Instead, use AC 2 to 9 as a damage cap. In fact, use the d20 roll as the damage done, up to a max equal to AC. That reduces combat to a single roll, instead of an attack roll followed by a damage roll. (For creatures that have a plus/minus to damage or that do 2 or more dice of damage, multiply or add to the damage after applying the cap. This is the only time math is required.) This does make combat a little deadlier for those wearing no armor. To incorporate level, use one of the following:
- If Level (HD) is higher than Dex, use Level (HD) instead (Swap Method.)
- If Level is higher than defender's Level, use 2d20 and keep the best result (Best of Two Method.)
- If the d20 roll is less than Level, you can ignore the damage cap (Open-Ended Method)
Under the Swap Method, Level won't matter for many fighters, and combat ability won't improve for a long time. Dex becomes more important. Best of Two is better, since it means even a 2nd level Fighter will have improved odds versus someone. However, it's a bit more "meta", which could be a turn-off. The Open-Ended Method doesn't change the combat odds at all, but it improves the chances of killing low-AC opponents.
Instead of rolling under Dex, you could roll under AC. Using d20 rolls, that makes everyone, even those wearing no armor, a little harder to hit. This does work well if you want to keep the standard d6 damage rolls, though. The Swap Method doesn't work well with this, and the Open-Ended Method is meaningless. Best of Two works, as does allowing the second critical if the roll matches Level instead of AC.
(If you are using Ascending AC and want to use this "elegant roll" approach, you should probably turn this into opposed rolls: attacker rolls under Dex, defender rolls under AC, highest wins.)
When judging the results of combat, the warnings about pass/fail checks are even more important to pay attention to. Remember, this kind of attack roll is a roll to kill, not a roll to hit.