A 28-year-old man is brought to the emergency department 2 hours after a flame burn injury sustained while cooking. On examination, he has erythema and blistering over his anterior chest, bilateral forearms, and anterior thighs. His face and neck appear red but blanch with pressure. Vital signs: HR 110/min, BP 130/80 mmHg, RR 22/min. What is the percentage of total body surface area (TBSA) affected by partial-thickness burns?