| Zaebos |
I hate to say it, Zaebos, but that kind of math can get a lot of characters killed. Two CR 13 monsters (Glabrezu, let's say) against a group of 6 10th to 11th leve characters is a really nasty challenge, especially as their ACs and Save DCs are callibrated for level 13 characters.
Now, if your party is very well equipped and has high stats and hp, they may well be able to take on such an encounter. But 4 of them in a row, without rest? I'm skeptical.
I think a *much* more accurate (and less lethal) system is to treat the effective party level the same way you'd treat a CR. If the baseline party is 4 characters of the same level, a party of 6 has a +1 EPL, while a party of 8 has a +2 EPL. This means to challenge Zaebos's example party, I would throw four EL 11 encounters at them, not four EL 15 ones.
I can't say that your system is more accurate, however, it is less lethal.
Your system is exactly like how you figure out Encounter Level. A party is not an encounter since they both measure different things.
Encounter Level should expend 20% of the party's resources.
So why would you calculate your party's level the same way?