Can you come point to 6.6666mm on my metric tape measure here?

Yes: For woodworkers, it's identical to 6.5mm, accurately eyeballed at between 6 and 5mm. Don't pretend you're a machinist. Does your tape measure even have a vernier scale. Does it make satisfying clicks when measuring. If you have a slip instead of interference fit just dump one piece in water for a second it'll be fine.