Could you give me a practical example of this, as I'm having trouble picturing it. The old brain ain't quite as sharp as it was once.
For example, if I wanted to make a monster where three of them was "worth" about 1 "regular" monster of the same level (or CR) or alternately, I wanted to make a monster that was "worth" THREE regular monsters, how ought I multiply them?
I'll start with a toy model.
The PCs do 100 damage per round. Monsters have 100 HP, and do 30 damage per round.
You fight 3 monsters. If the PCs lose initiative, the PCs take 90 + 60 + 30 = 180 damage.
To "win" initiative, they have to all 3 beat the initiative of the monster they chose to focus fire, killing it before it acts. That is relatively unlikely on round 1. There is a higher chance on later rounds.
Now we want a combo-monster "worth" 3 monsters to last as long. So we want 300 HP, give or take (X times the HP, where X is 3). The fight takes 3 rounds.
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Under my model, the 3x combo monster should do (1+X)/2 the damage of a single monster, where X is 3.
So the 3x monster monster does 60 damage per round. The PCs take 60+60+60 = 180 damage.
The idea is that this roughly lines up with the damage they take against the 3 separate monsters.
Basically. Surviving two encounters with a monster of your level would, in theory completely drain you of resources, if not take you down.
At least that was my initial thought - it may change!
a) Just dice will mean that a single encounter with monsters of your level is going to have a decent chance of a PC being dead, unless the PCs are optimized and "better than they should be". Like 10%. And PCs in 5e have to undergo 100+ fights to hit level 20.
Nobody gets to level 20 without "cheating" in the described system. You don't win 100s of close to even duels.
b) Variation of PC build power level is high enough that you'll generate a hard incentive to pick OP PCs. Less OP PCs will die a lot. If you then rebalance for said OP PCs, the process repeats, until you have at best a handful of viable PC builds.
c) 5e PCs are not designed around 2 encounter days. The endurance vs power output of PCs varies significantly between classes. A rogue's only daily and per-encounter resources is HD; in contrast, a wizard's resources are almost all on a per-day basis. Over a mere 2 encounters, the wizard can dump an entire day's worth of resources and be exhausted; the rogue, in contrast, is at worst a single large heal spell away from being 100% effective after the 2nd fight.
Draining the Rogue's resources precisely would require extreme fine tuning, and would fail at least half of the time, because the only resource you can drain is HP. And the Rogue can't "choose to spend HP". For the Wizard, they get to pick how fast they burn down their spells; depending on perceived danger, they are likely to "overkill" and burn too many spells rather than let themselves fall to single digit HP.
A fine-tuning that both forces the wizard to use all of their spells and drains both of the wizard and rogue's HD and drops them to single-digit HP is inplausible.