(centigrade in brackets) For an aircooled engine, 240 F (115) would be quite OK. 200(98) is low, 280 (140) is high.
Infrared guns are nice toys to compare things, but by no means precision instruments for laboratory use. You need thermocouples instead.
Having said that, that heat gun is very nice to check the cylinders of your twin. They very seldom will match, due to unequal mixture strength to both cylinders. It needs special care to have both cylinders within their margins. So if one cylinder approaches 280 degrees, it is time to richen the top end a bit more, or check ignition timing, compression ratio, spark plug, or whatever else influences that cylinder temperature.
Two points should be measured:
the junction between plug and cylinder head, and the first cooling rib on the cylinder. Take a mean of the two.
The plug measurement may be used to define whether you need a cooler or hotter plug type. The measurement is just the other way 'round as in glow plug engines. High temperatures require a colder spark plug. (in glow engines, high temps require a hotter plug)

All in all, it is purely relative, and largely dependent on engine type, amount of fin area, fin finish, engine shrouding , etc ad infinitum, so it is of very little use to state one single temperature as "best", as can be seen from the range provided at the beginning of this post.
Set your needles by ear first, then by reading the plug. Having done that, define temperatures for future reference and quick checkups.