1 1/2 turns out is supposed to be the generally accepted starting point...
I've read that you get the bike warmed up completely, then turn the screw all the way in (gently) then back it out 1/4 turn so it will start... start the bike up and adjust the idle speed screw (throttle stop) so that it idles at the lowest speed possible... Then start turning the airscrew out a bit at a time and listen for the idle speed increasing... when the idle speed won't get any faster or smoother by turning the screw out more, that is where you should leave it...
If you find this setting is much more than 2 turns out, then you have to go one step leaner on your pilot jet...
Good luck to you!
Feanor