The starting point in the manual is just that- a starting point. Lots of things influence what the proper setting will be on your engine including tank position, your choice of fuel, your choice of glow plug, ambient temperature and barometric pressure, and individual manufacturing tolerances. If this is a new engine, do the break in the way the manual says then set the high speed needle so that a pinch of the fuel line gives you a slight increase in rpm before the engine starts to shut down. Then set the low end so that the idle is steady for at least 30 seconds and then a push to full throttle gives you a smooth acceleration. Then recheck the high speed needle to make sure your tweak to the bottom didn't change it. If you'll do a search on "engine tuning" you'll find dozens of threads on the topic.