Good luck finding a detonation chart thats accurate. too many variables. I guess If you had a chart for one specific engine it would be semi accurate.
Just run premium. Its not going to cost you a whole lot more(we are talking mopeds here).
My rule of thumb has always been 10:1-13:1 premium, higher than 13:1, race gas or premium with an octane additive. I know there are a lot of motors out there that can get way with that compression without race fuel, but better safe than sorry especially on an aircooled engine. Just FYI, just cause you're not hearing "pinging" doesnt neccesarily mean your engine isnt detonating.