There are no official formulae for any NPC's released to date.
That said, the Gamemaster's Kit explicitly points out that NPC Nemesis characters do not use the PC generation mechanics unless the GM wants to do so. Further, nemesis character experience is handled differently from PC's when buying talents - they don't need to do so in order, and they all cost the same.
Remember the axiomatic definitions:
- Minions have no stress, no skills, and take stress/stun as wounds. In groups, damage from one rolls on to the next in that group.
- Rivals have no stress threshold, and take stress/stun as wounds. They are individuals. They are out of action from any critical.
- Nemesis have a stress threshold.
In general, I have noted that:
- Minions have about 50% of what a similar PC would have for WT.
- Rivals often have about the same WT as starting PC's, tho' some have up to about 6 more. Rivals skills are typically no more than level 3, often only levels 1-2.
- Nemesis levels have no less than starting PC levels of WT and ST. Attributes often are well above 150 points; skills are often in excess of level 2.
Balancing encounters is another matter entirely. There is no mechanical balancing included in any materials for Edge of the Empire nor for Age of Rebellion that I've seen.
Given the widely varied results of the dice rolling, mechanical balance is pretty hard to formulaically obtain.