The first thing to do is to lean how to read the temp gauge orrectly. What I mean is that you must be able to point the sensor directly onto the hottest part of the plug. If you miss, your readings can be as far off as 50degrees F!

As far as tuning and keeping within a certain temp range, different engines will have different temp ranges that they operate best at (as was already said in so many posts above). The best thing you can do is find out what temp ranges other are running at for the same engine in question. Also, ask them the fuel and exhaust they run and if they keep all head shims in or which ones they remove/add.

To say an engine should be kept to 240degrees F is falacious and disregards at least three oher variables. To begin to find waht operating temp range your engine is best at, check the owner's manual and set your engine up accordingly.
Warning: Alot of engine manuals are totally wrong. take what they say with a grain of salt until you can get your engine to perform its best without burning it up.
If you feel the engine can go a little hotter for better performance give it a try but with caution. For example, the engine manual tells you to keep max temps below 230degrees F but the engine does not turn at high RPMs for the high pitch scream (30000+RPM). You can turn the needle to get the engine to get that scream. You also might find out that the engine temp range is more like 250degrees F. From there, remember that engine sound and the needle settings. Also, keep a mental note of that temp range. This is the engine performance you should tune for given ideal outdoor conditions.