Glad you could reverse-engineer the save DC calculation. It is actually described in the SRD - and in several other places. Kept that one as an example.
Step 8: Special Abilities and Qualities
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.
So a special attack that doesn't already rely on a hit roll will have a save DC. Some special attacks that are particularly devastating might have both hit rolls and saving throw, but those are rare.
As Geoffrey points out in his answer, some spell-like abilities that emulate published spells don't use the 1/2 HD to calculate the save DC, instead using the spell level of the emulated spell for the DC calculation. Some unique (Sp) abilities still use 1/2 HD though.
Finally, Supernatural (SU) abilities ignore spell resistance, but spell-like (SP) abilities usually need a caster level (almost aways equal to HD, unless otherwise specified) to beat SR.