Hi!
Setting any engine is done by ear! That simple. This is something you just have to learn if you want to be successfull in R/C.
What you do is to start the engine at idle and then give full throttle and slowly turn the highspeed needle in so that the rpm goes up.
When you reach nearly max rpm you just back off a little (maybe 100-300rpm) so that the engine will rew slightly less than max rpm. Why you do this is because the engine rews up slightly in the air and needs the extra fuel.
Engines allways rew up slightly in the air due to the unloading of the prop when the air comes rushing toward the prop. When you set the highspeed needle slightly rich you give the engine a chance to rew up and reach its max in the air. This is how pylonracers as well as knowing sport pilots set their engines.
Never trust the smoke coming out of the silencer or pipe. People sometimes say "I set my engines so I see smoke coming out of the silencer" This method is too blunt and doesn't say enough how the engine "is feeling".
The only way exact way to set an engine is to listen to it, and act accordingly! And this is something you can learn.

Regards!
Jan K
Sweden