With the RB V12 I usually find that 4 turns out on the main needle (from fully closed/leaned), and 2 turns out on the low speed needle are good starting points from which to tune.
RB`s website also has some interesting points on it.....notably they make comment that tuning becomes more delicate if you use a fuel with greater than 16% nitro content.......and also that a 0.1mm shim be added for every 5% nitro you go higher than 16%.
Cheers