I set my idle as low as it will go and increase about 1/8 turn, with the engine running with the clutch disengaged. Each engine will differ, no set RPM like in a car. No timing light needed.
Use your ears...these motors sound ugly when you overrev them...as far as idle, I just go with what the engine seems to like. Not worth the time modding a light IMHO. I run mine at the "sweet spot" rpm's.
Adjust the idle speed just high enough that it doesn't stall when the throttle is released. Why worry about what RPM that is? It's unimportant.
There are tachometers available but they're not going to provide you with any meaningful information. Set your idle speed as low as you can get it then go ride and have fun.
Tom