[For just a few moments, it's all Sherlock can do to stare. Is this how John looks when Sherlock issues a deduction, all open mouthed and foolish? But then his surprise melts into a sudden approving grin.]

Exactly right!

[See, he knew that John must have picked something up watching him all those years.]

I couldn't have put it better myself. I mean, I could have put it the same way after a fraction of the time, but-- Well done, John. Well deduced.