A 62-year-old man with moderate ARDS is ventilated with VCV, TV 6 mL/kg IBW (420 mL), PEEP 10 cmH2O, FiO2 0.5, RR 22/min. Plateau pressure is 25 cmH2O. ABG: pH 7.36, PaO2 9.8 kPa, PaCO2 5.5 kPa, HCO3 23 mmol/L. The PaO2/FiO2 ratio is 147 mmHg. He is on noradrenaline 0.08 mcg/kg/min. According to the ARDSNet PEEP/FiO2 tables, what is the most appropriate adjustment?