If I were to be a Peri-blooded Aasimar who get's pyrotechnics as an SLA how would i determine the DC for it? I can't find the equation anywhere.
It's explained in the Universal Monster Rules in the Bestiary.
The saving throw (if any) against a spell-like ability is 10 + the level of the spell the ability resembles or duplicates + the creature's Charisma modifier.
And if you end up with an SLA that multiple classes get at different levels:
Some spell-like abilities duplicate spells that work differently when cast by characters of different classes. A monster's spell-like abilities are presumed to be the sorcerer/wizard versions. If the spell in question is not a sorcerer/wizard spell, then default to cleric, druid, bard, paladin, and ranger, in that order.