Yeah look at the guide for weight and use on the SRAM site as a starting point, but sag setting is best way. I'd go more 25-30% on the trail end of things cos I'm not a race snake like Oz. GMBN have a good vid on doing it if you stuck
Maybe I am late but... as it is a debonair (bigger air chamber) you can go up to 30%. But that can be to much for a 120 mm travel. I'd try how it does fell with pressures between 15 and 30 % sag and choose what you like better: more climbing ability or more small bumps sensibility. In my case with 125mm travel and a RT3 I went for 25-30% SAG