I suspect that you won't notice ANY difference in how the bike runs.
Without a wide-band air/fuel ratio gauge, this will have to be done by trial and error. Don't worry, you won't hurt anything in your engine by guessing too lean under such light load conditions.
Cut it down by 10% at 0, 2, 5, 10% throttle and 5% at 20% as a first guess. Probably you'll notice no difference, maybe throttle response might even be a bit better and for sure the fuel consumption will improve. If it's okay at that setting, try another 10% at 0,2,5,10 and 5% at 20 (bringing your total reduction to 20% and 10% respectively). At that point, there is the possibility that you might start noticing flat spots or hesitation at certain engine speeds. If not ... give it *another* 10 and 5 in the same manner.
Sooner or later you are going to notice certain engine speeds or throttle positions where it just feels flat and unresponsive. You'll know it when it happens. This means that you have gone too far. But since the setting before that was "known good" ... backtrack and increase fuel *at those specific engine speeds only* by (say) 4% at 0,2,5,10 and 2% at 20% throttle.
Eventually, by trial and error, you will find the leanest-smooth-running settings throughout the RPM range that you normally use, which still gives good throttle response and without driveability issues.
Expect a big improvement in fuel consumption and range, and your oil won't smell like gas, and your spark plugs won't look black or wet with fuel any more.