OK, so I'm adding a spellcaster class to a monster that normally is considered a combat role monster. The monster's base CR is 6. If I add 5 or less levels to the monster, I understand that its CR increases +1 every two levels, but once I add that sixth level of spellcaster, do all that levels added count at +1 CR per level, or only the ones added once the class level matches the CR?
Classes that are marked “key” generally add 1 to a creature's CR for each level added. Classes marked with a “—” increase a creature's CR by 1 for every 2 class levels added until the number of levels added are equal to (or exceed) the creature's original CR, at which point they are treated as “key” levels (adding 1 to the creature's CR for each level added).
I'm inclined to think the levels below the CR cut-off are always treated as non-key and only the ones equal-to or above the CR are counted as key levels, but I'd like to know how it's normally handled.
My reading of this is that the class levels above the original CR become +1 per level, so any "-" levels up to 6 would be at +1 per 2, then if you add the 7th class level, and any other class levels beyond, they would be rated at +1 per level. So, in the case you're talking about, a CR 6 combat type monster, with 6 levels of a spell class added, would be a CR 9 creature. That's my understanding of that language anyway.
You are inclined correctly.
The first six levels count half (for a total CR of 9).
The seventh level, and all subsequent ones, are counted as 'key', raising CR by one each.
A CR 6 monster with 5 HD that is combat role that you add 6 levels spell casting class is CR 9.
Base CR 6 + 2CR for (first 5 level of spell casting class)+ 1 CR (1 level of spell casting class)= CR 9
Note after you advanced any monster you should check it vs table here
http://paizo.com/pathfinderRPG/prd/monsters/monsterCreation.html
Or table 1-1 page 291 Bestiary to make sure it line for it CR.
Advancing a moster is an art.. Not a science... Results may vary
Thanks guys. And yeah, I'll try to make sure it's in-line for its CR via the table too.