The OP's vehicle IS within spec/tolerance and yes, it would be difficult even with today's technology to get the speedo dead accurate without using a GPS. Unless your tires always have the same pressure which means its always the same temperature, you travel at only one speed and the rubber on your tires never wear down, it is nearly impossible. Tires even change diameter at different speeds.
Yes, it SHOULD be as simple "plug into computer and press a few buttons" thing for the dealer to do but unfortunately it is not for the LX470. If anybody can prove otherwise, I'd surely be grateful to know.