High speed needle IMO is the most difficult needle to tune.
You can only fine tune this by ear and feel...in other words you have to tune a few engines to learn how it should be.
In general you want it to rev clean without gurgling from too much fuel.
I mean you can hear an engine that runs rich...it doesn't rev high either. Start leaning it by 1/8ths at first until it starts to make good power and rpm but it still sounds rich (gurgles) and there is enough smoke coming out the exhaust tip.
Then start leaning by 1/12ths at a time cause this is the stage of fine tuning. You just want to tune at the point where it revs freely and doesn't gurgle at higher rpm. Although a small gurgling is acceptable in my opinion since it keeps temps down and rpms also (protects the engine from overeving and running lean).
Idle, should be as low as possible without dying when idles for 15-20sec.
It keeps the engine cool and saves fuel.
I hope this helps you out...