4.5-5" from piston face is a good start. The gas temperature is quite consistent at this position, and is the temperature I use as data for pipe design.

6"+ will show a higher temperature for the same setting.

It depends if you're designing pipes using specific software that requires the temp at a certain point, or setting the jetting, in which case you're after a constant number that you can work to.

You're looking for the temperature that suits your engine. this will be just below it's peak figure. Then you look to jet for that number.

If you don't reach the number then you're too rich, beyond is too lean.

If the temp rises beyond and then starts to drop back, there's a good chance that it's detonating and best to shut the engine off before it destroys itself.

Advancing the ignition will increase combustion chamber temperature, but EGT will drop.

Retarding the ignition will reduce combustion chamber temperature, but EGT will rise.