I prefer this encounter calculator:
Encounter EL & XP Calculator 4.01
Mostly it's the same code, but it does seem cleanly organized to me, and with lots of explanatory text.
Anyway, regardless of which calculator you use, the rest of the gang here is correct: with 10 characters every calculator you find will tell you they can handle extremely difficult challenges, but it's not always true.
For example, the calculators we've linked to all say that you could pit them against 5 CR 5 enemies as a "Very Difficult" challenge. That's 5 young dragons, for instance. That's a recipe for everyone to die within 2 rounds. So don't trust the calculator for stuff like that.
Instead, as others mentioned, use the calculator to figure out a reasonable match for *many* low CR challenges. If you use the calculator to see if the PCs can handle one CR 6 enemy, the calculator will be off. But if you use the calculator to see if the PCs can handle six CR 2 enemies, you'll get a more accurate read.
High CR enemies will have high-damage attacks that insta-kill low level characters. So instead of powerful single or paired enemies, pit them against groups of low-CR enemies. I would probably cap it at CR 4 for now, and when they hit level 3, I'd cap it at CR 5, and when they hit level 4 I'd cap enemies at CR 6. After that, it's
far more open-ended. 10 level 5 characters will probably be able to overwhelm many difficult foes (assuming there are a couple level 5 wizards or clerics in the party, as they will have gained access to level 3 spells, which brings the group to a whole new tier of power).
Your bigger problem is what happens at the real-world table with 10 players. You need to realize that if you limit each player to only 3 minutes for his or her turn, that still means each player will have to wait nearly
thirty minutes to take a turn. If even 1 or 2 players starts in with rules questions or has extra attackers to run (from animal companions or the leadership feat or from summoned critters), it's going to bog down
fast. You will end up with people literally placing their heads on the table and sleeping.
So if I were you, I'd establish rules right up front: if you don't have your actions ready to go on your turn, you suck for delaying everyone else; if you take more than 2 minutes, you suck for delaying everyone else; if you create a character that has lots of minions, you suck for delaying everyone else, etc. Just MHO, of course. Good luck!