For the purpose of flying basic fixed-pitch 1970s/1980s Yank spamcans without any posh stuff like CHT, EGT or FF gauges, analysers etc., I was taught to lean until the RPM rises slightly (<50RPM) then when it starts to fall again enrich the mixture slightly.
It is unusual that so many people aren't taught the basics of leaning and managing power settings, or seem to be completely ignorant of them. Flying around on full rich all day isn't very efficient and doesn't do the engine much good either.
Smithy