The trouble is that this type of aircraft is probably not over-equipped with engine instrumentation.
The full rich mixture is set to provide enough fuel to retard the combustion event sufficiently to cool the engine at full-power sea level conditions.
When climbing (say 5000ft) the non-turbocharged engine cannot make full power anymore, so less rich mixture is required. However in the absence of a JPI engine analyser this is going to be difficult to judge how much to lean.
The trick is to gradually lean the mixture during the climb to maintain a constant EGT temperature. This will provide better climb on less fuel since too rich mixture will reduce horsepower and waste fuel.
Have a look at this presentation to understand the concept.
http://www.advancedpilot.com/leaning.pps
In descent, keep the mixture lean, not full rich, to avoid lead fouling of the spark plugs. It is perfectly OK to go to peak EGT in descent.