In my experience, the challenges to a group is as much a function of their overall ability scores, equipment and player resourcefulness as their actual level. It is my belief that the challenge rating system fail miserably when: 1) players have too high of ability scores (i.e., more than 28 point purchase); 2) players have too much equipment (i.e., beyond the recommended starting gold for a given level or too concentrated in "cheaper" expendable items, when they are willing to use them); or 3) when players who otherwise would be well-adapted to any situation just suddenly draw blanks.
Depending on how you have chosen to run your game, you may find that the standard challenge (i.e., EL = Level of characters) to be NO challenge at all. The other way around is usually rare and mainly due to point 3. After all, most people do not assign less than 24 points for ability scores and do not provide less than half of the items in gold value suggested.
Your formulaic approach is also distorted. After all, a 6-member level 2 party facing an EL 3 challenge is very different than a 6-member level 12 party facing an EL 18 challenge, who can cast 9th level spells at them. You may want to consider bumping your EL's by 1 or 2 or even 3 instead of multiplying. But then again, you may find your party's ability scores/equipment allows them to handle something much higher than that. One last thing, an encounter of the same EL as the players' levels is supposed to drain HALF of their resources. Chances are...that isn't happening in your campaign. That should clue you in as to what effective player level they really are. Just bear in mind that you shouldn't necessarily up the challenge to that level, as most players will not enjoy being beaten to within an inch of their lives constantly. Hope that helps.
