There is no problem with leaning in the climb. Taking off from some high altitude fields requires leaning before take-off!
You can find the altitude at which the engine is no longer able to develop 75% power in the engine handbook.
If you have a Cylinder Head Temperature gauge you set up a stable cruise and just lean until it peaks. This is the setting you want.
With an Exhaust Gas Temperature gauge the technique is almost the same except that you want to increase the mixture again to 25 degrees F rich of peak temp. for best economy or 75 dgrees F rich of peak temp. for best power.
It is easier to understand the process and thereby get dispell the myths and mysteries if you remember that all the fuel which doesn't get burnt (used economically) is used for cooling. Ergo, the point at which there is no excess fuel in the combustion chamber means there is no fuel for cooling and the highest temperature will be achieved.