A 75-year-old man with COPD is admitted with an acute exacerbation, presenting with increased dysnoea, increased sputum volume, and sputum purulence. His oxygen saturation is 89% on air, respiratory rate is 26/min, pH 7.32, PaCO2 8.1 kPa, PaO2 7.5 kPa, HCO3 29 mmol/L. According to NICE NG115 (COPD), what is the most appropriate initial oxygen therapy target saturation range?