There is far too much going on at idle for there to be any single answer.
Header surface area, compression, header thermal conductivity, cam overlap, flow restriction, mixture, fuel consumption, and ignition timing all have a large influence.
Street driving my car without excessive under hood pipe and turbo heat required a stoichiometric mixture and around 28-30 degrees advance. The main thing I worried about was all the heat from the pipes and turbo.
Its a tricky question to answer. I fought with this in the past and tried several IR temp guns. Ended up buying a Fluke adjustable one (pretty sure its this one and bought add on temp wire plug-in Contact Thermometer | Fluke 561 Infrared and Contact Thermometer ). Was told 10 different things by 5 different people it seemed. Coated vs uncoated vs which coating, 1inch from head vs straight vs bend, cold vs running temp, idle vs mid throttle vs full load, list went on. Biggest thing was to look at it like a compression test. Look for something different/out of place, keep track of any changes. With that said there is a general temp range that should be acceptable. I don't have my records with me but 400F seems around norm. Of course, in a perfect world individual EGT probes would be best.
In my experience it has proven to be a quick an easy tool and was capable of finding a broken valve spring.
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