Most of the time when an engine is new and you can't get it to idle it's because the idle is to low and the needles are too rich. Preheat it like you're doing and make sure it's still that temp when you try and start it. The head dissipates heat very fast so it may be worthwhile to heat the engine continuously for several minutes so the heat "soaks". Vary the distance of your heat gun so the temp doesn't get to high. 100°c is fine. Use the throttle trigger to help get it started. Now if it starts and stays on, good. If not, turn your idle up six hours (screw it in). Lean the top and bottom 3 hours. Check your engines temp and try again. If it stays on now, but sounds like the idle is too high or doesn't sound rich, richen the needles an hour at a time and lower your idle. It should be rich at this point. Keep the heat gun pointed at the side of the head to maintain 100°c. If it doesn't stay on and you need to turn the idle up more and lean it out more, go ahead. Get the thing started and running and then you can richen it back up and lower the idle as needed. Don't mess with the mid speed needle (right side of engine on the carb slide). It's not that big a deal. You'll get it. "The best method" for break in its a while other thing. Keep in mind that if the engine doesn't start after a couple of attempts your glow plug could be bad. All coils must light especially the first one.