Determining DCs for Su and Ex abilities

[trentonjoe]

Is there a formula for determining the DC for monster's Supernatural and Extraordinary abilities?

For example:

A gibbering mouther's Spittle attack (Ex) is a Fort save of 13. How does that number get choosen?

In addition to that if the mouther is increased form 4HD to 15 HD does the DC go up?

Thanks in advance.

 

[pontus]

Usually it's either 10 + Con Modifier + HD/2 (for stuff like poison, disease and other physical stuff), or 10 + Cha Modifier + HD/2 (for magic-like thingys like gaze attacks, energy drain etc).

 

[trentonjoe]

Thank you!

I've never been to Sweden but it sure seems to have a lot of knowledgeable D&D players.