ORIGINAL: downunder
Another way to get a "close enough is good enough" initial idle setting is this:
1. Put a long length of tubing on the fuel nipple.
2. Set the barrel slightly open about where the engine would idle.
3. Open the main needle 2 or 3 turns or if you've had it running at full throttle then leave it there.
4. Go somewhere quiet.
5. Blow through the tubing and listen carefully for air going through the carb.
6. Adjust the idle needle in or out until there's just a faint hiss of air.
It seems that it is also a great way to get close enough with the advantage that you are saving fuel (which now costs a leg and an arm for those who likes higher nitro contents).