Both- - - -center the octane selector and tighten that bolt at the zero point, then loosen the clamp screw and set the ignition timing that way. Then the octane selector can be used to fine-tune the timing if necessary. Back when gasoline octane numbers were not as standardized as they are today, vehicle owners could tweak their timing a little if a tank of gas produced a detonation rattle. Now it's not likely you'll have to do that.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!