This is the best site I have seen for reading spark plugs and just thought I would pass it on.

How to read spark plugs.

 

I spoke at length with a tech from DynoJet. I was asking him jetting questions and mentioned the plug-chop. They firmly believe that this doesn't method of checking how the engine is running to be inconsistent. He said that because some brands of fuel are more oxygenated than others, this allows different plug readings under similar conditions. He said it wasn't a viable test to use.

Just his $0.02, I guess.