karolh, I think it is simply not that critical. If you are to the point that you need every last rpm that an engine will turn then you may need a larger engine.

I richen my engines by about 200 rpm from peak which is usually about 1/8 turn and have never had a problem with lean runs. The spark plug also runs a nice tan color.

I don't think you will find any information more precise than this, only different opinions.

Ken