EL for 20 Aboleths AND NOW level of 7 17th level characters?

[Ridley's Cohort]

Don't know if this might help, but when I have a six-character- party, I usually double the normal encounter for a four-character one. Usually works fine since damage dealing spellcasters usually get more foes in area spells.
At least when they're dealing with monsters rather than NPCs.
Can be tricky with complicated mixed encounters though.

We found that a double sized party (8 PCs) can handle triple the number opponents of a standard 4-person party. A tactically savvy party can protect the spellcasters and rotate badly wounded PCs off the line. You also see fewer catastrophic runs of bad luck, e.g. 4 of 4 PCs failing the save versus a Sound Burst.

 

[Mark_Aurel]

Someone correct me if I'm wrong.

To find Effective Party level add together all of your characters levels, in this case 7 x 17. Then divide the number by 4. This gives you 29.75. This seems a bit high but 7 17th lv characters get 7 actions (or more with time stop and quickened actions) per round vs a standard parties 4 actions.

That wouldn't be entirely accurate, given the way things scale. Take a fighter as an example. Going up in level, his attack value will increase, his damage output will tend to increase, his AC will increase, and his hit points will increase. Assuming that two fighters of the same level have 50/50 odds, a fighter that's just two levels higher will have overwhelmingly better odds against his lower-level opponent. Assuming a fairly typical improvement in equipment, he'll be about +3 ahead in terms of attack, +2 or +3 ahead in terms of AC, 12-18 hit points or more ahead, and perhaps +1 ahead in terms of average damage per attack. All those things add up to quite an increase in potency - the tendency is somewhat geometric when you break it down and look at the game in terms of probability of success on the d20. Using the above numbers, an xth-level fighter will hit an xth+2-level fighter on a 12 or better, while the xth+2-level fighter will the xth-level fighter on an 8 or better - about 56% more often, for more damage, and against a smaller hit point pool.

A party of 17th level characters is NOT taking on any 29.75 CR monsters, though they might be able to take on a horde of lower-level monsters which theoretically adds up to that (as in, a zillion orcs does add up to it).

Now, by virtue of their number of actions, you might actually see a 7-member 17th-level party as somewhat more formidable than a 4-member 19th-level party, depending on the opponent in question. Generally speaking, though, the CR formula indicates that CR+2 = 2× as difficult as CR, which would make 7 17th-level characters roughly equal to 4 19th-level ones, and thus eligible for EL 19 encounters. It gets very iffy at that level, though.