I reently saw an article about assigning Combat Values in Against the Darkmaster as a way balancing encounters, and I am now working on a rating system of my own that will also go into this
(the sytem will also appear in an issue of expres Additions as well, along with the rating for all of the creatures in the core rules, plus the rating for the 16 pre-gens and creatures from additional products).
The Encounter Rating (or Combat Rating, still deciding on name) system is actually pretty easy
Level + Defensive Modifier + Primary Attack Bonus + 1/5 Secondary Attack Bonus*
* this only applies if more than one attack per round is possible