I would believe the NGK chart, no disrespect meant to your mechanic, but I would think that the plug manufacturer knows a little more about reading plugs. Even with the newer gas it is a little spazzy at times, but generally speaking about halfway or more down the ceramic is where I read mine. That's why I jetted again last night, mine was running too rich (black plugs) and my mileage sux, only about 30-35mpg. I went from 160 down to 150 mains to try and get a few more miles for my hard earned pence, but not at the cost of performance decreases either. Feels just the same as before. I can swap my jets in about 15 minutes now, getting really good at taking them apart by now.
