Pretty much what Hobbsy said although I use a thermocouple to check head temps on the test stand but only for interest's sake. When an engine is tuned correctly (meaning slightly rich and definitely not lean) then the temp will be what it will be. What the temp gauge can be useful for though is comparing the temps on a particular engine when changing something like oil content or fuel brand. With an infrared thermometer you have to be careful to always aim at the same place and the base of the glow plug is the best spot because virtually all engines have the same thickness of metal at that point.