How to figure up monster's DC for abilities?

[killem2]

~these~ emit a piercing scream when frightened. Creatures within 100 ft. of a screaming ~this~ must make a Fortitude save (DC 14) or be deafened for 1d6 minutes and dazed for 1d6 rounds. Creatures who successfully save are still deafened for 1d6 rounds. A creature whose ears are covered gets a +4 circumstance bonus on its saving throw. The save DC is Constitution based.

Well, its constitution decreased from 12 to 8, it has 2 HD.

Where is the formula for these?

[Biffoniacus_Furiou]

10 + 1/2 HD + ability modifier.

In this case, 2 HD and 12 Con would be a base DC 12 (1/2 of 2 is 1, the ability modifier for 12 is +1), and it probably has Ability Focus (http://www.d20srd.org/srd/monsterFeats.htm#abilityFocus) or a racial ability which gives it +2 DC for the total 14.

Reducing its Con from 12 to 8 would be a net loss of two (+1 to -1), so the DC would be reduced by two.