Short version: If you are still picking up power and the timing mark is getting into the threads the plug is to hot. If you can't see any timing mark and you know you are in the ball park for timing or power is falling off then your plug is too cold. This assumes you have the fuel dialed in and are not octane limited.
Basically you have to tune to what the plugs tell you and then look at the other indicators to see if something isn't adding up. Hopefully that makes sense because I am not good at explaining things like this.
This DVD has a good section on how to read plugs at the end of it and will get you headed in the right direction. It was done on a carbed engine but if you listen to the process you can apply most of it to EFI tuning as well.http://www.stevemorrisengines.com/steve-morris-engines-store/how-to-dyno-and-tune-your-boosted-engine-dvd.html
Digital Download available too:http://www.stevemorrisengines.com/steve ... nload.html