There is little to recommend lean and mean in a small a/c, engines are too expensive, fuel not so much.
The Lycoming guides are really useful for this and should be read by all who operate aircraft engines. There is a section on leaning and recommendation of procedures as well as the reasons to do so. One is extended engine life.
Just looking at the thread subject though, if you take the performance table for the aircraft you are flying, copy it into a spreadsheet and add another column that calculates miles/gallon at given performance setting/altitude then you can clearly see the trade-offs for the various engine settings. Plot the range, endurance, TAS, mpg in a graph and voila. This has to be done for each aircraft type, there is no formula as such.
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