There's no set figure for head temps because it can vary wildly from engine to engine. The only way you can do it for a particular engine is to use a known good fuel, start it and tune it by ear or with a tacho then check it's temp for that particular day's temperature. If it's a hot day it'll run hotter and if it's a cold day it'll run cooler. Basically, if you can tune by ear then there's no need to use a temp gun (or a tacho).