[moneymanager]

The Bosch and NGK numbers do change in opposite directions.
I believe you want the coldest plug in your car that will not foul. The higher the state of tune, the more desirable this is as it tends to reduce detonation/pre-ignition possibilities. But there's way more analysis going on here than is necessary. Pick an NGK or Bosch plug and put it in. All of the plugs mentioned here are close to what you want. Take them out and check in 100 miles. NGK has a lovely site showing you how to read plugs... not rocket science. They are either cold, or hot, or spot on, and it's easy to tell. As you get the car tuned over time to where you like it, see if you can go one plug colder.