Without instrumentation, the method for correct-ish leaning is to set it all up in cruise, 75% of max rated power or less, and then slowly lean until there is a fairly sudden drop in power (or airspeed if you like). Then advance (enrich) the mixture back just a little, to the point before the sudden power drop.
With this method, I actually get about 20 litres/h on my 172M at 2200 revs (~ 50 %) with leaning at around 5000 feet - official figure is 23 l/h (so about 15 % fuel saving on chart value and about 20+ % on unleaned - I tried). I'm getting a JDI 700 with fuel flow meter soon though.