I'm trying to balance some combat encounters involving a large number of NPC's and enemies. I'm wondering if there is some rough formula using attribute level for this? For instance, one attribute level 6 creature is equivalent to three attribute level 2 creatures, or something like that.

I cannot answer your question, yet I will give you a word of caution:
the “attribute level” isn't only a measure of the creature's power, but the measure of its “power-up” too.

Look at the Hobgoblin: attribute level 4, like an Orc Guard.
Yet with “denizen of the dark”, favored Armour (2d+4/8), favored Axe (3d+4/8), “horrible strength” (+4/+8 to damage) and endurance 36 it's a fearsome opponent. Far stronger than an Uruk (attribute 5) or an Orc Chieftain (attribute 5). Let alone “no quarter”, that makes him a real heroes-killer

Attribute 4 is a measure of its bonus, but it's dangerous as an attribute 6 creature (a stone troll is more durable, yet less able to inflict death on the fellowship).

Hope it helps.

I posted this some time ago. It was just an idea then and I haven't worked further on it since then, but it might help to craft a viable encounter.

TOR XP Value Guidelines

If you want to find the original topic (there wasn't much discussion on the subject) I would suggest looking up the link name.

Hope this is of some help to you.