Once the broth is boiling, turn off the heat and immediately add the garlic and chilies. Season with the sugar, fish sauce and lime juice, then taste and add more fish sauce if needed. (How much fish sauce depends on how far your stock reduced, so you have to taste and adjust here.) When ready to serve, stir in the cilantro.
3 Tbsp lime juice, 1 Tbsp fish sauce, 2 tsp sugar, 3 Tbsp chopped garlic, ⅔ cup chopped cilantro