Cylinder head temperaturs on an air cooled engien are a guessing game, usually. It's almost impossible to get an accurate reading after landing because the engine has cooled down, and the same holds true for bench-run engines, because the cooling airflow is not the same as that inside the cowl of a plane. A lot of people are quoting optimal temperatures in the 185-200 degree range, measured at the top of the cylinder near the spark plug, which is the hottest part of the engine, but this is after landing usually.

Best bet is to just duct the cowl properly, provide a good large exit for the airflow, and keep an eye on your fuel mixture. More engines are damaged by running too lean than by getting too hot in a properly ducted cowl. You can do comparative readings on the engine to see how the cooling process is coming along, but in reality it's very difficult to say that "X" temperature is best.

Kris Welter