I think I've heard people say generally not to go above 180F for any screen. But I think that's even a bit high. When I use my heat plate to separate LCD's, it's set to 60C(140F). Personally, for using a heat gun I go by touch to see if it's hot enough. I know it's not exact, but for me, if I can keep my finger on the screen for about 3 seconds before it burns, then I know it's perfect. I've been using that method for a couple years now, and on hundreds of screens. I would say just don't break 160-170F and you should be just fine.