Where exactly do I find the formula for computing a monster ability's DC?
For a spell-like ability, it's right there in the Bestiary, p.304, under "Spell-Like Abilities". There is no such general entry for Supernatural Abilities or Extraordinary Abilities, however.
Many monster abilities are spelled out in the Appendix, and state plainly how that specific ability's save DC is calculated.
But some monsters have abilities that aren't spelled out. (For instance, an astral deva's stun ability, or a cockatrice's petrification ability.) For this abilities, it simply states the DC, and which ability score it is based on.
I have been unable to find the general formula spelled out anywhere, and this is bugging me. I know it's "10 + half HD + ability score modifier". I've used it for a long time. But I cannot for the life of me FIND it.
Please note, I am not looking to have the general formula explained...I am looking to have its location pointed out to me.
As an FYI, my main reason for this is to see if save DCs increase when adding class levels. Specifically, does the general formula state "+ half HD" or "+ half racial HD"?
I have seen instances in official Pathfinder adventures where it does, and also where it doesn't, so no help there. :(
I'm pretty sure this is also listed somewhere else (which I can't find at the moment), but in the "Monster Creation" section, under "Step 8: Special Abilities and Qualities", this is written:
Most special abilities that cause damage, such as breath weapons, give a save (Fortitude, Reflex, or Will depending on the ability). The DC for almost all special abilities is equal to 10 + 1/2 the creature's Hit Dice + a relevant ability modifier (usually Constitution or Charisma depending on the ability). Special abilities that add to melee and ranged attacks generally do not allow a save, as they rely on the attacks hitting to be useful.