The best way to set idle needle is, AFAIK, by pinching the fuel line. Let the engine run idle for some time (>30 seconds) and then pinch the fuel line shortly. If the engine speeds up, it's too rich. If it dies, it's too lean. If you don't get any change, it's ok. After having done this, reduce throttle as far as possible. No tachometer needed for this, just listen to your engine. After having done this, redo the pinch method again. Once you think you got it right, let the engine run idle for some time (>30 seconds) and then quickly advance throttle to full. It should gain RPM without any hesitation. If it quits immediatly, it's probably too lean. If it stalls for a second and then picks up again, it's probably too rich. If it does this, let the engine run idle again (>30 seconds) and then advance throttle to full quickly. The second it stalls, pinch the fuel line. If that helps, it's too rich. If not, it's too lean.

Using a tachometer for idle is nonsense. Sure you can say 'I want an idle RPM of 2200', but if your engine won't run reliable at those speeds, you can use all the tachometers you want, it won't help.

A tachometer is usefull to see the max RPM an engine gets with a certain prop. For instance; I'm using a Saito 82a fourstroke and had a 13*7 prop on it. After adjusting the needles, I measured top RPM, which turned out to be around 11500 RPM. Considering this is on the ground and the RPM will be higher in the air, this is an indication this prop is slightly too small. I replaced it with a 13*8 prop and now i'm getting like 10500 RPM, which will translate to around 11500 in air (max RPM for this engine is about 12k).

Also remember to run the engine slightly rich on the ground, or else it'll die on you in the air.