ATOW has rules for making NPCs in the Gamemastering section starting on page 336. Rather than building them by XP, you build them based on the character's build.
For instance, a low level "Thug" type character, they recommend in Attributes having 10-15 points fewer than the PCs, for Traits they recommend 2-5 fewer than the PCs and they should have 2-5 fewer skills, which should be 1-2 levels below the PCs.
So if you have PCs that on average have spent 35 points on their attributes, you should assign 20-30 points to the Thug-level NPC, emphasizing the NPCs usage (so a mafia "thug" would have the same number of attribute points as a doctor "Thug" but the mafia NPC would probably focus on combat related attributes, while the doctor would have higher attributes that are linked to medical related skills.)
Traits are similar, though you probably don't need to structure them as much for low-level NPCs. Just give them something that might be useful for their intended roll. Higher level NPCs might need a bit more structure, since they might do more for the NPCs.
Skills again should focus on what the NPC does. If its going to be a fight, just give him some combat skills a the correct level (for example, Thug-level should be 1-2 points below the PCs).
So basically ATOW suggests making your NPCs based on the level of the NPC (Thug, Soldier, Savant, Boss) and the level of the PC.
For NPCs created by the Enemy trait, you may wish to get more involved in the building of them, but random mooks should be pretty simplistic and stick to what you want them to do for the party.
