I play D&D 3.5. I have a party of 6 players and myself as a DM. In my "career" as a DM, I seem to have no to little difficulty with adding monsters and encounters. But I have never understood how the Encounter Level system works. What encounter level can my party handle, and what makes that level? (How many monsters of what CR?)
I suggest using an ECL calculator to determine how much your group can handle. Remember this is just a guide and some monster's CRs are not very accurate, and some groups will be more effective than others, thus requiring harder ECLs. D20 ECL Calculator is a good free calculator that even does some additional calculations for you.
Once you know how to calculate the ECL the DMG offers guidance on what to strive for. Again this is generic and may need to be altered if your group is better than avg.
From the DMG, pg 49-50.
10% Easy EL lower than party level
Okay, here's my rough rule-of-thumb rules for figuring encounter level "adequateness".
And I have my own rules of thumb for calculating an Encounter Level.
So an Ogre (CR 3) with six Orc minions (6 times 1/2 matches the Ogre's CR) makes for an EL of roughly 4. A party with an adjusted average party level of 4 should make it through okay. A party at 2nd level, however, will find the damage output of that ogre challenging and you're going to risk a party death, even though my formula would indicate it as a "reasonable-but-high" encounter.
I have one additional caveat I'd like to make on my formula: it doesn't scale well for the PCs. We have a big group (our parties are typically 8 characters). The problem with large parties is that scaling encounters gets increasingly difficult with size. At 8 characters, I need to scale my encounters a couple CR to bring on any kind of challenge. Doing so, I have to be hyper vigilant about damage output, because my players are "fragile". This makes sense, if you think about it. I mean, a horde overcomes tough obstacles, but only by sacrificing some members while doing so. Encounters at the average party level are speed bumps. Encounters at the adjusted party level (+2 in this case) have the potential to wipe someone out if damage gets concentrated poorly.
It sounds like you are an experienced DM, who has been successful so far without using the EL/CR rules.
That being the case, I don’t think they’ll help you.
Monsters’ CRs are often terribly inaccurate. Monster Manual II is particularly egregious, such as the CR 9 Adamantine Horror that has at-will Disjunction. Dragons are under-CRed almost by tradition. Numerous monsters are actually pushovers for their CR, particularly monsters whose CR depends primarily on their relative lack of vulnerability.
Perhaps the best example is the Allip (a 4 HD, CR 3 Undead) and the Tarrasque (a 48 HD, CR 20 Magical Beast). The Allip is incorporeal, and the Tarrasque therefore cannot touch it. Meanwhile, the Allip deals Wisdom drain, which is one of the few things the Tarrasque is not immune to. A fight between an Allip and the Tarrasque ends with the Tarrasque unconscious, every time. (or the Tarrasque running away from the scary ghost, but the description of the Tarrasque suggests that it’s literally incapable of considering retreat; being an Int-3-incarnation-of-rage-and-destruction will do that)
The Allip is under-CRed; few 3rd-level parties have a good way to deal with incorporeal enemies, and Wisdom drain is phenomenally dangerous. The Tarrasque, though, is over-CRed – while it’s very difficult to hurt, it does have vulnerabilities, and for 20th-level characters, protecting oneself from the Int 3, grounded, and corporeal Tarrasque is not a great challenge.
Actually, since a low-level Cleric can rebuke Allips, and a low-level party can pool their funds for a Candle of Invocation, for a smart/optimized party, the Tarrasque could literally be closer to CR 3 than Allips are...
And then there is the fact that party power varies immensely. A 20th-level Wizard can stop time (time stop), has the most powerful creatures of the multiverse at his beck and call (gate), and can literally create his very own demiplane (genesis). A 20th-level Monk... can use feather fall at-will, but only if he’s next to a wall. These are not particularly close.
So CR rules aren’t very useful. Monsters have wildly differing power levels, even at the same CR. And parties have wildly different power levels, even at the same EL. Unfortunately, the best a DM can do is simply know his party and know his monsters, and try to find a certain level of compatibility.
General Rule: a party of 4 characters at level X can be expected to win an encounter of level X using a moderate amount of resources. Three or four of those encounters in one day should exhaust the party. One monster at CR X produces an EL (Encounter Level) of X.
Adding a second monster of the same CR usually increases the EL by 2. I usually somewhat fudge the EL's when adding more monsters. For instance, adding a CR 1 monster to a CR 6 monster really doesn't change the EL at all (still stays at 6). Adding seven CR 1 monsters to a CR 6 monster may bump the EL up to 7.
Adjusting from the above is somewhat non-trivial, particularly when changing the number of characters from 4 to something else or having the characters' levels differ by more than 1 or 2 from each other.