I know that doubling a set number of monsters adds +1 (for example, changing from 3 kobolds to 6 makes an encounter go from CR 1 to CR 2, or having two human rogue 3s is a CR 4), but what if I'm using different monsters?

I have an encounter that I want to make a CR 3 using a kobold sorcerer 2 and skeletons. A skeleton is CR 1/3, so do I need 3 skeletons or 6?

Snarky answer: You need 0 skeletons and a few things actually worth their CR.
Non-snarky answer: 3, as best I can tell, although the table on p49 isn't terribly clear when dealing with such awful creatures.

At least you gave me a place to look, which is more than I had. Thanks, OTTer man!

I use this encounter calculator. It's not perfect, especially at the low end, but it's good enough.

Thanks! I noticed what you meant: a CR2 + 3 CR1/3 is a "Very Hard" EL3, but dropping one of those CR1/3s makes it an "Easy" EL3.

One thing it did make me change my mind on was the last encounter. Originally I was going to have a CR3 acquired wererat/rogue, a CR2 natural wererat, and 2 CR1/2 Fiendish Dire Rats for a EL 5 (APL is 3). That looked like it was going to be WAY too hard, so I think I'll use the FDRs in a different encounter.

Skeletons can be useful. Sure, wights and such are outright better, and zombie dragons are a much better use of the Animate pool, but you don't get nearly as many, and a dread necromancer's skeletons aren't as bad for their HD. They clear traps, and an army of NPC redshirts is roughly on the same level. Of course, if you intend to actually challenge a party, I'm in agreement.

I'm actually more confused as to why a sorcerer would use castings of Command Undead on a bunch of 1hd skeletons, now that I think about it.

Or why a sorcerer would waste spells known on it in the first place.

He could have found a knowstone or something. A level 2 knowstone is within level 4 NPC WBL, if I remember correctly.

Still. Why not just pawn it off, then? A sorcerer can do better things with the money.

You know, I actually have no idea how CR works at all.

~SE++

It's a very imprecise system. I never really figured it out, and just eyeball it.

Better things than buy Knowstones? Bull, although this particular spell is lulzily bad.

That said, the sell price of a level 2 knowstone will only let them get a level one knowstone, so I honestly wouldn't blame a sorcerer for holding onto it until they afford to move up, given how powerful this spell is in it niche.
Neither did the writers. Just eyeball it and call it good.

Well, they could buy a level one Knowstone with a more generally useful spell.

Yeah, that's the general sense I've gotten. Seems the consensus is to just keep the encounters somewhat reasonable and reward EXP based on performance and how difficult you see it (or just award based on what percent of the current level you think they did).

~SE++

Point, but there is merit in buying a low-level knowstone of a situational but powerful spell like that. Admittedly, scrolls are simpler, but for a spell you'd expect to be casting a lot when using it, it's something that would be reasonable to hold on to. This is a perfectly reasonable knowstone to have, assuming you got it as loot or something.

I suppose.

Plus, what level one spell would a sorcerer put in a knowstone that it doesn't already have?