Jan- while you are technically correct you are still wrong in a way. You can tell when a Saito is too rich by the smoke as Saitos are know to not smoke much when set just right. The reverse is true with most OS 4 strokes. This doesn't tell you what is set right, but it does tell you when you are too rich or too lean depending on engine brand.

I like to use a tach when setting the high speed needle valve. What everyone leaves out of the tuning process advice is the plane isn't finished with tuning until you fly it around and listen to the engine. If it has a smooth transition, doesn't bog down or stumble after setting it 300 rpm rich from peak then it's tuned.