I'm going to have to agree with Dicks' 200f number. And with 180c, or 392f, being much too high. As for using a heat gun "immediately" after landing, the engine has had the benefit of a reduced throttle setting and a light engine load for the duration of the approach. Either by themselves is enough to cool an engine substabtially in 30 seconds after either is initiated. Just reducing the throttle to idle from full on the ground can decrease an engine temp by 80 degrees or more in the fist 10 seconds. Add to that the dubious accuracy of any of the heat guns and you will find that even using one as fastideously as possible to be more of a "feel good" thing than anything else.