As you may be aware, I believe that there are implictily several "tiers of play." For sake of this exercise, I'll break them up into these categories:
Low levels: 1-4 HD
Medium levels: 5-8 HD
High levels: 9-12 HD
Very High levels: 13+ HD
First we need to define our "average" characters.
The FIGHTER is in plate mail and bearing a shield (AC 17) with +1 HP/HD.
The CLERIC is in chain mail with a shield (AC 15).
The MAGIC USER is nekked except for minor spells (AC 11).
At low levels, all arms and armor are normally mundane and spells are rare. By medium levels, we can expect +1 gear to have shown up. By high levels, +2 gear at a minimum is standard. At very high levels, +3 gear is out and about. Magic-users normally don't benefit from magic arms & armor, but let us assume that their defenses likewise improve at higher levels as well due to availability of spells and such.
Additionally, at higher levels, protective magics are highly likely to be increasingly available, even if it is just a simple Protection from Evil spell. Thus, at mid levels, we'll assess another +1 bonus and at high levels an additional +2 bonus.
So, that gives us this little chart:
LOW LEVELS: +1 (+1 armor OR +1 spells)
MID LEVELS: +2 (+1 armor/+1 spells)
HIGH LEVELS: +3 (+2 armor/+1 spells)
VERY HIGH LEVELS: +4 (+3 armor/+1 spells)
We can now define the AC and HP of an average character of pretty much any level.
LOW LEVELS (say, level 2): AC 18, HP 9
MID LEVELS (level 6): AC 19, HP 26
HIGH LEVELS (level 10): AC 20, HP 45
VERY HIGH LEVELS (level 14): AC 21, HP 63
For clerics, just subtract -2 from the AC figures and -1/level. So:
LOW LEVELS (say, level 2): AC 16, HP 7
MID LEVELS (level 6): AC 17, HP 20
HIGH LEVELS (level 10): AC 18, HP 35
VERY HIGH LEVELS (level 14): AC 19, HP 49
Now for "average" monsters.
Monsters get +1 to hit for every HD.
They tend to deal 1d6 damage base.
Monsters at mid-to-high levels deal 2d6 damage (Djinn, 7 HD; Efreeti, 10 HD; Elementals, 8/12/16 HD; Giants, 8-12 HD; Giant Slugs, 12 HD; Treants, ~7 HD).
High and Very High level monsters can deal 3d6 damage (Balrog's aura, 9 HD; Earth & Water Elementals, 8/12/16 HD; Cloud Giants, 13 HD; Storm Giants, 16 HD; Rocs, 12 HD; Treants, ~10 HD)
Very, very few monsters deal 4d6 damage.
So here are our threats.
LOW LEVEL: +2 to hit for 1d6 damage
MEDIUM LEVEL: +6 to hit for 1d6+2 damagee (like an ogre)
HIGH LEVEL: +10 to hit for 2d6 damage
VERY HIGH LEVEL: +14 to hit for 3d6 damage
Now we can construct a table showing the number of rounds each character will last vs. a given threat.
FIGHTING MAN: AVERAGE DAMAGE SUSTAINED PER ROUND AND AVERAGE ROUNDS OF SURVIVAL
| || || |
| || || |
A "level appropriate" encounter is in blue. An easy encounter is in green. And a hard one is in red.
You could construct one for clerics and MUs, but all it would show is the number of rounds of survival decreasing. It'd be a big drop for both, because clerics have only around 2/3 as many HP and about -10% to HP; MUs have significantly worse AC and many fewer HP.
Interestingly, the "baseline" is for a fighting man to be able to take 10 attacks. Also interesting is that at low levels, combat is very swingy. The fighting man can take 10 attacks because he is very unlikely to be hit, but once he is hit, he's likely to die in one or two blows. At higher levels, the fighting man relies on his HPs, not his AC.
This is because the accuracy of monsters increases faster than character defenses (monsters get +1/HD, players get +1/4 HD), but player HP scales up faster than monster damage.