well i test mine with a laser temp gun. aim it just beside the glow plug in the engine heatsink.
the normal temp here in kamloops bc is very hot in the summer and super DRY! about 104 on a good cool day.
and my engine does not run above 190. and she is running good.
i have had it upward of 250 but it is usless for my onroad. too fast for anything around.
so i tone her down and richen her up. got awsome (off the line)
it all really depends on many things that will make your engine run good.
i cant wait till the winter. it gets around -30 here easy. dont know what it will run like then.