A 48-year-old woman with ARDS is ventilated with VCV, TV 360 mL (6 mL/kg IBW), PEEP 14 cmH2O, FiO2 0.7, RR 28/min. Plateau pressure is 30 cmH2O. The team performs a decremental PEEP trial. At PEEP 10 cmH2O, compliance improves to its highest value (40 mL/cmH2O) and then deteriorates at PEEP 8 cmH2O. Following an open-lung recruitment manoeuvre, what PEEP level should be set?