You can find all about tuning and glow plugs at Master Dennis Ritchey's web site:

http://www.richeyracingengines.com/tuning.htm

In short, one must tune the engine for max. performance and watch out that a clear trail of smoke comes out of the pipe after every acceleration coming out of turns, and that power doesn't decrease over the lenght of the tests, then read the plug, correct tuning if necessary.
After that, when you feel comfortable with performance, take a temp reading and register it for reference purpose.
Every engine and ambient condition has it's "ideal" ref. temp, but one musn't tune for temp, but for performance.
All new engines are comfortable running (racing) between 250°F and 260°F.

AFM