and the "aiming point" should be the plug boss. make sure it is not a shiny surface. Dull black is best. This kind of measurement only is useful when running the engine on a test stand.
The other cheap option would be a venom temperature gage. (use search option). Position the sensor in the airflow shade, against the cylinder wall, underneath the topmost cooling fin. This device records, and holds the max operating temperature so you can read it after landing.
The one and only option is a professional metallic temperature sensor that is underneath the spark plug, using a recording system like many 2.4gHz radios have nowadays.