The head is the hottest and best location to measure. I use the intake manifold because it is more conveneint and I like seeing lower temperatures. The sensor is in direct contact with the water and thus the headers should not adversely affect the readings as long as the sensor housing is not touching the headers.

The boiling point of 50/50 EG water is 265F at 15 psig. As long as you are reasonably under this (220F), not boiling off coolant, and your idle mixture and timing suit the temperature you should be OK.

It is certainly preferable to operate cooler (190F full time would be great) but it may just not be practical. A 220F head reading might read 200F or so at the intake? This would be considered acceptable for me.

Kitz