Rat, you are dead on with your heat range explanation.
4 years ago I was out on the salt flats was honored to witness old ISKY himself tuning a fresh 500 cid blown Keith Black Hemi in a fuel roadster. He walked me through his tuning process and he spent most of his time on heat range versus how many threads were burnt on the plug. On that engine he wanted to see 4 threads burnt.
I have always used the two thread rule for heat range. I like to see color in the two top threads of a plug. On all the forced induction stuff I do I try to use as cold of a plug as I can without fouling to add a little margin of saftey.